Create ANNOUNCE-github.md

First attempt to define the organization, please comment!

etc/ANNOUNCE-github.md

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+# The Mathematical Components github organization
+
+We are plase to announce that the development of the Mathematical Components now
+happens in the public and takes advantage of github's organization system.
+
+http://math-comp.github.io/math-comp/
+
+The purpose of such organization is to create a synergy between the Mathematical Components
+library and its ecosystem of related projects.
+
+The main archive is now avilable as a git repository in the organization name space
+https://github.com/math-comp/math-comp and accepts bug reports and pull requests via
+the standard github interface.
+
+Users of the library are encouraged to join the organization an place (or mirror) their projects
+in the organization name space.  The immediate gain is discoverability.  The more long term deal
+we propose is that the impact of non backward compatibile changes in the Mathematical Components
+library will be carefully assessed on all the mature projects part of the organization.  In
+exchange we expect members of the organization to actively maintain their code when a breaking
+change is applied or a patch is proposed.
+
+The organization hosts a wiki for exchanging tips and tricks, gotchas, good practices, etc:
+https://github.com/math-comp/math-comp/wiki
+
+The ssreflect mailing list shall serve as the privileged communication channel between the
+organization members: https://sympa.inria.fr/sympa/info/ssreflect
+
+-- The Mathematical Components team