Add message about move to GitHub

README.rst

 |pipeline status| |coverage report| |pypi| |conda| |version|
 
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-Bilby
+Bilby development has moved to `GitHub <https://github.com/bilby-dev/bilby>`__!
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+Please open any new issues or pull requests there. A full migration guide will be provided soon. Links below here may no longer be active.
+====
 
 A user-friendly Bayesian inference library.
 Fulfilling all your Bayesian dreams.