docs(README): move to .rst extension

Hello dear contributors,

This is an auto generated modification to help you improve the code base of logilab. If this commit is bad don't hesitate to close this MR and repport it.

Kind regards,

MANIFEST.in

@@ -2,3 +2,4 @@ include *.py
 recursive-include data external_resources *.gif
 recursive-include i18n *.pot *.po
 recursive-include migration *.py
+include README.rst