[readme] Add link to information about release plans.
[readme] Add link to information about release plans.

This is a proposal to use the wiki to gather current information about
the release process.

ejgallego/coq,Zimmi48/coq,silene/coq,JasonGross/coq,gares/coq,JasonGross/coq,ppedrot/coq,coq/coq,SkySkimmer/coq,coq/coq,silene/coq,Matafou/coq,ejgallego/coq,Matafou/coq,gares/coq,JasonGross/coq,ejgallego/coq,Zimmi48/coq,gares/coq,Matafou/coq,ppedrot/coq,ppedrot/coq,gares/coq,ppedrot/coq,SkySkimmer/coq,ejgallego/coq,coq/coq,ppedrot/coq,Zimmi48/coq,silene/coq,gares/coq,SkySkimmer/coq,Zimmi48/coq,SkySkimmer/coq,coq/coq,JasonGross/coq