README: Git for Windows is currently switching to the Git for Windows SDK

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Remove the dysfunctional link to tip4commit

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

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge pull request #285 from dscho/readme

Update README.md