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 <johannes.schindelin@gmx.de>
Merge pull request #14 from dscho/readme

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