Update the link to the documentation

It requires at least that we must use `dune-release` for the release (and push the generated documentation into the gh-pages branch) but it's the usual and current process.

README.md

@@ -31,7 +31,7 @@ are being resolved to.
 
 ### Further Informartion
 
-* **API Docs:** http://docs.mirage.io/
+* **API Docs:** https://mirage.github.io/ocaml-conduit/
 * **WWW:** https://github.com/mirage/ocaml-conduit
 * **E-mail:** <[email protected]>
 * **Bugs:** https://github.com/mirage/ocaml-conduit/issues