Licensing/Copywrite Language for POUF

[hexsecs]

We reference...

Copyright
Each POUF MAY either be explicitly labeled as placed in the public domain or licensed under the Open Publication License.

There are several problems with this statement.

1. https://edtechbooks.org/openedreader/open-public-license
   Open pub license is now been retired and licensors are now encouraged to use Creative Commons licenses.
2. An OEM may not want their POUF implementation to be public domain or licensed under a defunct license.

I propose we update the POUF readme to reference Creative Commons and only recommend that POUFs are open sourced, acknowledging that an OEM may not want to publicly disclose their POUF.

[JustinCappos]

Sounds good. Do you want to create a PR for this change and I'll review / merge it?