Clarification of licensing requirements

Licensing requirements under the big tent have been unclear at best.
The Foundation bylaws impose some of them, but their interpretation
was left as an exercise to the reader. The TC traditionally came up
with rules for dependencies (through the requirements repository
README) but those were mostly unchecked. Finally, we need flexibility
for the project infrastructure, which should be able to run any
open source solution that fits.

After much back and forth with the Foundation legal counsel on what
could be acceptable under the bylaws, we agreed that the following
proposal was acceptable, and compatible with the current situation.

I propose we add it as a reference document to our governance
repository so that it's easier to point prospective projects to it,
and to track changes in the list of acceptable licenses over time.

Change-Id: I7ed11388e43bd5fcf2e8e6595c353429300387d5

reference/index.rst

@@ -10,6 +10,7 @@ Reference documents which need to be revised over time.
    charter
    projects/index
    new-projects-requirements
+   licensing
    service-project-naming
    project-testing-interface
    irc

reference/licensing.rst

@@ -0,0 +1,30 @@
+========================
+ Licensing requirements
+========================
+
+Official OpenStack projects need to follow a number of rules when it comes
+to licensing. This document clarifies what those requirements are.
+
+In order to be considered for inclusion in the **TC-approved release**
+(the subset of OpenStack projects that may be included in a Defcore
+trademark program), the project must be licensed under ASLv2.
+
+**OpenStack projects and libraries produced under the Big Tent** should
+generally be licensed under ASLv2, and otherwise must be licensed under
+a license supported by the CLA, which allows redistribution under ASLv2
+(currently limited to MIT and BSD (both forms)). In particular, service
+projects are recommended to always pick ASLv2 so that they may be included
+in the TC-approved release in the future.
+
+In order to be acceptable as dependencies of OpenStack projects,
+**external libraries** (produced and published by 3rd-party developers)
+must be licensed under an OSI-approved license that does not restrict
+distribution of the consuming project. That includes ASLv2, BSD (both
+forms), MIT, PSF, LGPL, ISC, and MPL. That excludes GPLv2, GPLv3, and AGPL.
+
+**Projects run as part of the OpenStack Infrastructure** (in order to
+*produce* OpenStack software) may be licensed under any OSI-approved license.
+
+Other licenses (not explicitly listed in this document) may be considered
+in the future on a case-by-case basis by the Technical Committee, with the
+help of the OpenStack Foundation legal counsel.