Merge branch 'add_docs_to_readme' into 'master'

Add link to docs to the Readme

See merge request iris/stdpp!42

README.md

-# Coq-std++
+# Coq-std++ [[coqdoc]](https://plv.mpi-sws.org/coqdoc/stdpp/)
 
 This project contains an extended "Standard Library" for Coq called coq-std++.
 The key features of this library are as follows: