Adjust README.md to reflect the new Git for Windows home

Git for Windows switched orgs, to reflect the fact that msysGit is no
longer used to develop the project, but instead the Git for Windows SDK.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

README.md

@@ -1,6 +1,6 @@
 # Git for Windows
 
-This is the source code of [Git for Windows](http://msysgit.github.io/),
+This is the source code of [Git for Windows](http://git-for-windows.github.io/),
 forked from [Git](http://git-scm.com/).
 
-If you encounter problems, you can report them as [GitHub issues](https://github.com/msysgit/git/issues?direction=desc&sort=updated&state=open), discuss them on Git for Windows' [Google Group](http://groups.google.com/group/msysgit), and encourage others to work on by tipping via [![tip for next commit](http://tip4commit.com/projects/295.svg)](http://tip4commit.com/projects/295).
+If you encounter problems, you can report them as [GitHub issues](https://github.com/git-for-windows/git/issues), discuss them on Git for Windows' [Google Group](http://groups.google.com/group/git-for-windows), and [contribute bug fixes](https://github.com/git-for-windows/git/wiki/How-to-participate#fix-bugs-or-add-features-in-the-git-code-itself).