Update documentation for git.

README.git

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+Before you can start compiling tinc from a fresh git clone, you have
+to install the very latest versions of the following packages:
+
+- OpenSSL
+- zlib
+- lzo
+- GCC
+- automake
+- autoconf
+- gettext
+
+Then you have to let the autotools create all the autogenerated files, using
+this command:
+
+autoreconf -f -s -i
+
+If you change configure.in or any Makefile.am file, you will have to rerun
+autoreconf. After this, you can run configure and make as usual. To create a
+tarball suitable for release, run:
+
+make dist
+
+To clean up your working copy so that no autogenerated files remain, run:
+
+git clean