docs: Clarify licensing rules for the project

[lattner]

LGTM with a typo fix

llvm/docs/DeveloperPolicy.rst

928

want to contribute and email [e-mail] requesting a review.

[kristof.beyls]

llvm/docs/DeveloperPolicy.rst

924–925

Not sure if "LLVM Foundation Board of Directors" is better than "LLVM Board of Directors"?

[tstellar]

Typo fixes.

llvm/docs/DeveloperPolicy.rst

 
 This section addresses the issues of copyright, license and patents for the LLVM
 project.  The copyright for the code is held by the contributors of
 the code.  The code is licensed under permissive `open source licensing terms`_,
 namely the Apache-2.0 with LLVM-exception license, which includes a copyright
 and `patent license`_.  When you contribute code to the LLVM project, you
 license it under these terms.
 
+In certain circumstances, code licensed under other licenses can be added
+to the codebase.  However, this may only be done with approval of the LLVM
+Foundation Board of Directors, and contributors should plan for the approval
+process to take at least 4-6 weeks.  If you would like to contribute code
+under a different license, please create a Phabricator review with the code
+you want to contribute and email [e-mail] requesting a review.
+
 If you have questions or comments about these topics, please contact the
 `LLVM Developer's Mailing List <mailto:[e-mail]>`_.  However,
 please realize that most compiler developers are not lawyers, and therefore you
 will not be getting official legal advice.
 
 Copyright
 ---------