started to implement sequence container

docs/begin.md

@@ -0,0 +1,41 @@
+```cpp
+iterator begin();
+const_iterator begin() const;
+const_iterator cbegin() const;
+```
+
+## Description
+
+Returns an iterator to the first value in the JSON container. If the JSON container is empty, the returned iterator will be equal to [`end()`](https://github.com/nlohmann/json/wiki/nlohmann::basicjson::end).
+
+![illustration of iterators](http://upload.cppreference.com/mwiki/images/1/1b/range-begin-end.svg)
+
+## Parameters
+
+None.
+
+## Return value
+
+Iterator to the first value. Note the return value its deferencabilty depends on the different value types:
+
+| value type | deferenceable | description |
+| ---------- | ------------- | ----------- |
+| null       | no | `null` has no value, always equal to [`end()`](https://github.com/nlohmann/json/wiki/nlohmann::basicjson::end) |
+| boolean    | yes | iterator to the boolean value |
+| string     | yes | iterator to the string value |
+| number     | yes | iterator to the number value |
+| object     | only if object is not empty | iterator to the begin of the object |
+| array      | only if array is not empty | iterator to the begin of the array |
+
+## Complexity
+
+Constant, as long as `ArrayType` and `ObjectType` satisfy the [Container concept](http://en.cppreference.com/w/cpp/concept/Container).
+
+## Exceptions
+
+None. The function's noexcept-specification is `noexcept`.
+
+## See also
+
+- [**end**, **cend**](https://github.com/nlohmann/json/wiki/nlohmann::basicjson::end)<br>
+  returns an iterator to the end