Added squash note to contribute.md

Because this requirement is not listed anywhere, but was mentioned in https://github.com/ipfs/go-ipfs/pull/2202#issuecomment-171876804.

License: MIT
Signed-off-by: Richard Littauer <richard.littauer@gmail.com>

contribute.md

@@ -90,3 +90,8 @@ reasons why this is _the right thing to do_:
 License: MIT
 Signed-off-by: Juan Benet <juan@ipfs.io>
 ```
+
+### Each Commit Must Pass Tests
+
+All commits in a PR must pass tests. If they don't, fix the commits and/or [squash them](https://git-scm.com/book/en/v2/Git-Tools-Rewriting-History#Squashing-Commits) so that they do pass the tests. This should be done so that we can use git-bisect easily.
+