Remove the dysfunctional link to tip4commit

Due to the lack of donations, tip4commit removed us from their list.

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

Author: dscho

README.md

@@ -8,4 +8,4 @@ development environment](https://github.com/git-for-windows/) and that the
 upcoming Git for Windows 2.* will be maintained
 [here](https://github.com/git-for-windows/git).
 
-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/msysgit/git/issues?direction=desc&sort=updated&state=open), discuss them on Git for Windows' [Google Group](http://groups.google.com/group/msysgit), and for bigger tasks, hire developers via the mailing list.