Splitting up the README files

Moved git related information over to README.git,
and polished the README file slightly.

Change-Id: I863e2c2ef9c2020964185e96ddb87a05d82c30a6
Reviewed-by: Rohan McGovern <rohan.mcgovern@nokia.com>

README.git

@@ -0,0 +1,55 @@
+OBTAINING QT5 FROM GIT
+======================
+
+ Synopsis
+ ========
+
+ For a more in-depth description on how to build Qt5 from git,
+ please see: http://qt-project.org/wiki/Building-Qt-5-from-Git
+
+
+ Get The Submodules
+ ==================
+
+ Cloning Qt5 does not automatically clone all submodules.
+ To get the submodules, it is recommended to use the `init-repository' script.
+
+   ./init-repository
+
+ See `./init-repository -help' for full documentation on the many supported
+ options.
+
+
+ Update The Submodules (optional)
+ ================================
+
+ After a plain `git clone' and `init-repository', you will have each submodule
+ checked out to a particular SHA1.  This combination of SHA1s is guaranteed to
+ have passed some basic acceptance testing.
+
+ If (and only if!) you need newer versions of some submodules, you can manually
+ fetch and/or pull and/or reset any modules you like, or use the qt5_tool to
+ pull all modules:
+
+   ./qtrepotools/bin/qt5_tool -p
+
+ NOTE! As soon as you change the SHA1 of any submodule, you are using a
+ combination of modules which is potentially untested and broken!  If you want
+ something which is more likely to compile, stick to the module SHA1s provided
+ in this repository.
+
+
+ Install The Git Grafts (optional)
+ =================================
+
+ If you want to be able to connect the Qt5 and Qt4 development history, you
+ may set up git grafts with the following command:
+
+   git submodule foreach '../qtrepotools/bin/git-qt-grafts <path_to_history>'
+
+
+ Developing and Contributing to Qt
+ ================================
+
+ For more information on how to develop and contribute to Qt, please see:
+ http://qt-project.org/wiki/Category:Developing_Qt