[DeveloperPolicy] Add invitation link for commit access.

[python3kgae]

Summary

In some case, GitHub will not send notification for commit access invitation. Add invitation link for people don't get notification from GitHub.

[lattner]

Nice, thank you!

llvm/docs/DeveloperPolicy.rst

* When re-applying a reverted patch, the commit message should be updated to
   indicate the problem that was addressed and how it was addressed.
 
 Obtaining Commit Access
 -----------------------
 
 We grant commit access to contributors with a track record of submitting high
 quality patches.  If you would like commit access, please send an email to
 `Chris <mailto:[e-mail]>`_ with your GitHub username.  This is true
-for former contributors with SVN access as well as new contributors.
+for former contributors with SVN access as well as new contributors. If
+approved, a GitHub invitation will be sent to your GitHub account. In case you
+don't get notification from GitHub, go to
+`Invitation Link <https://github.com/orgs/llvm/invitation>`_ directly. Once
+accept the invitation, you'll get commit access.
 
 Prior to obtaining commit access, it is common practice to request that
 someone with commit access commits on your behalf. When doing so, please
 provide the name and email address you would like to use in the Author
 property of the commit.
 
 Your first commit to a repository may require the autogenerated email to be
 approved by a moderator of the mailing list.