Document how to set commit-msg hook for plugin submodules

The commit-msg hook in .git/hooks is not used when making commits
on gits that are included as git submodules.  The hook also needs
to be installed in the submodules' hooks folder.

Update dev-contributing to mention this.

Change-Id: I8cfc2eb30409e113a1c1757fbe4629875067d334
1 file changed