contribution guidelines (#175)

CONTRIBUTING.md

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+# How to contribute
+
+This project started as a little excuse to exercise some of the cool new C++11 features. Over the time, people actually started to use the JSON library (yey!) and started to help improving it by proposing features, finding bugs, or even fixing my mistakes. I am really [thankful](https://github.com/nlohmann/json/blob/master/README.md#thanks) for this and try to keep track of all the helpers.
+
+To make it as easy as possible for you to contribute and me to keep an overview, here are some few guidelines which should help us avoid all kind of unnecessary work or disappointment. And of course, this document is subject to discussion, so please [create an issue](https://github.com/nlohmann/json/issues/new) or a pull request if you find a way to improve it! 
+
+## Prerequisites
+
+Please [create an issue](https://github.com/nlohmann/json/issues/new), assuming one does not already exist and described your concern. Note you need a [GitHub account](https://github.com/signup/free) for this.
+
+If you want propose changes to the code, you need to download the [`re2c`](http://re2c.org) tool.
+
+## Describe your issue
+
+Clearly describe the issue:
+
+- If it is a bug, please describe how to **reproduce** it. If possible, attach a complete example which demonstrates the error. Please also state what you **expected** to happen instead of the error.
+- If you propose a change or addition, try to give an **example** how the improved code could look like or how to use it.
+- If you found a compilation error, please tell us which **compiler** (version and operating system) you used and paste the (relevant part of) the error messages to the ticket.
+
+## Files to change
+
+There are currently two files which need to be edited:
+
+1. [`src/json.hpp.re2c`](https://github.com/nlohmann/json/blob/master/src/json.hpp.re2c) (note the `.re2c` suffix) - This file contains a comment section which describes the JSON lexic. This section is translated by [`re2c`](http://re2c.org) into file [`src/json.hpp`](https://github.com/nlohmann/json/blob/master/src/json.hpp) which is plain "vanilla" C++11 code. (In fact, the generated lexer consists of some hundred lines of `goto`s, which is a hint you never want to edit this file...).
+
+⋅⋅⋅If you only edit file `src.json.hpp` (without the `.re2c`) suffix, your changes will be overwritten as soon as the lexer is touched again. To generate the `src.json.hpp` file which is actually used during compilation of the tests and all other code, please execute
+
+...    make re2c
+
+...To run [`re2c`](http://re2c.org) and generate/overwrite file `src/json.hpp` with your changes in file `src/json.hpp.re2c`.
+
+
+2. [`test/unit.cpp`](https://github.com/nlohmann/json/blob/master/test/unit.cpp) - This contains the [Catch](https://github.com/philsquared/Catch) unit tests which currently cover [100 %](https://coveralls.io/github/nlohmann/json) of the library's code.
+
+...If you add or change a feature, please also add a unit test to this file. The unit tests can be compiled with
+
+...    make
+
+...and can be executed with
+
+...    ./json_unit
+
+...The test cases are also executed with several different compilers on [Travis](https://travis-ci.org/nlohmann/json) once you opened a pull request.
+
+Please understand that I cannot accept pull requests changing only file `src/json.hpp`.
+
+
+## Please don't
+
+- Only make changes to file `src/json.hpp` -- please read the paragraph above and understand why `src/json.hpp.re2c` exists.
+- The C++11 support varies between different **compilers** and versions. Please note the [list of supported compilers](https://github.com/nlohmann/json/blob/master/README.md#supported-compilers). Some compilers like GCC 4.8 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual Studio 13.0 and earlier are known not to work due to missing or incomplete C++11 support. Please refrain from proposing changes that work around these compiler's limitations with `#ifdef`s or means.
+- Specifically, I am aware of compilation problems with **Microsoft Visual Studio** (there even is an [issue label](https://github.com/nlohmann/json/issues?utf8=✓&q=label%3A%22visual+studio%22+) for these kind of bugs). I understand that even in 2016, complete C++11 support isn't there yet. But please also understand that I do not want to drop features or uglify the code just to make Microsoft's sub-standard compiler happy. The past showed that there are ways to express the functionality that the code compiles with the most recent MSVC - unfortunately, this is not the main objective of the project.
+- Please refrain from proposing changes that would **break [JSON](http://json.org) conformance**. If you propose a conformant extension of JSON to be supported by the library, please motivate this extension.
+
+## Wanted
+
+The following areas really need contribution:
+
+- Getting the code to compile without errors with the lates **Microsoft Visual Studio** version. I am not using Windows myself, so I cannot debug code with MSVC myself. There is a job on [AppVeyor](https://ci.appveyor.com/project/nlohmann/json) though.
+- Extending the **continuous integration** beyond Linux running some versions of GCC and Clang on [Travis](https://travis-ci.org/nlohmann/json) and Microsoft Visual Studio on [AppVeyor](https://ci.appveyor.com/project/nlohmann/json). We found a lot of bugs just because several compilers behave in a slightly different manner.
+- Improving the efficiency of the **JSON parser**. The current parser is implemented as a naive recursive descent parser with hand coded string handling. More sophisticated approaches like LALR parsers would be really appreciated. That said, parser generators like Bison or ANTLR do not play nice with single-header files -- I really would like to keep the parser inside the `json.hpp` header, and I am not aware of approaches similar to [`re2c`](http://re2c.org) for parsing.
+- Extending and updating existing **benchmarks** to include (the most recent version of) this library. Though efficiency is not everything, speed and memory consumption are very important characteristics for C++ developers, so having proper comparisons would be interesting.