git/README.md: Update link to SWC contributing page

Catch up with the swcarpentry/website -> swcarpentry/site transition,
specifically 86ba191 (Replaced contributing.html page with contrib/
directory containing separate pages for different ways to contribute,
2013-08-22).

diff --git a/git b/git
index 03ceaf942632835aeb84f7525f178bd9cc002b07..ad0c8b89d9f134462df11bd80a0e2a8a61a0a08d 160000 (submodule)
--- a/git
+++ b/git
@@ -1 +1 @@
-Subproject commit 03ceaf942632835aeb84f7525f178bd9cc002b07
+Subproject commit ad0c8b89d9f134462df11bd80a0e2a8a61a0a08d