Add information for potential contributors

CONTRIBUTE.md

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+Hagrid is owned by the [p≡p foundation] and licensed under the
+terms of the AGPLv3+.
+
+  [p≡p foundation]: https://pep.foundation/
+
+To finance its mission, privacy by default, the [p≡p foundation]
+allows third parties (currently only [p≡p security]) to relicense its
+software.  Consistent with the rules of a foundation, the money
+collected by the foundation in this manner is fully reinvested in the
+foundation's mission, which includes further development of Hagrid.
+
+  [p≡p security]: https://www.pep.security/
+
+To do this, the [p≡p foundation] needs permission from all
+contributors to relicense their changes.  In return, the
+[p≡p foundation] guarantees that *all* releases of Hagrid (and
+any other software it owns) will also be released under a GNU-approved
+license.  That is, even if Foo Corp is granted a license to use
+Hagrid in a proprietary product, the exact code that Foo Corp
+uses will also be licensed under a GNU-approved license.
+
+If you want to contribute to Hagrid, and you agree to the above,
+please sign the [p≡p foundation]'s [CLA].  This is an electronic
+assignment; no paper work is required.  You'll need to provide a valid
+email address.  After clicking on a link to verify your email address,
+you'll receive a second email, which contains the contract between you
+and the [p≡p foundation].  Be sure to keep it for future reference.
+The maintainers of Hagrid will also receive a notification.  At
+that point, we can merge patches from you into Hagrid.
+
+  [CLA]: https://contribution.pep.foundation/contribute/
+
+Please direct questions regarding the CLA to [contribution@pep.foundation].
+
+  [contribution@pep.foundation]: mailto:contribution@pep.foundation