Added first run of opam-pin blog post

A call to action for beta users would be helpful here perhaps also with a link to the opam issue tracker.
(sidenote: I had a quick look for a filter for beta4 bugs but didn't see one. Is the [1.2 milestone](https://github.com/ocaml/opam/issues?q=is%3Aopen+is%3Aissue+milestone%3A1.2) the correct one?)