README.md: update Mac OS X instructions

I'm going to stop uploading the git-cola.app tarballs
so make the instructions for creating them easier to find.

Signed-off-by: David Aguilar <davvid@gmail.com>
doc: update v1.9.2 release notes

Signed-off-by: David Aguilar <davvid@gmail.com>
git-cola v1.9.2

Signed-off-by: David Aguilar <davvid@gmail.com>
win32: update installer output path

Signed-off-by: David Aguilar <davvid@gmail.com>