Explain how to get certain release using repo

[erikboto]

It could be nice to add a small note in the section discussion repo about how you can fetch a certain release tag using the -b flag, e.g.:

repo init -u https://github.com/Pelagicore/pelux-manifests -b refs/tags/v1.0/pyro

[tobsan]

We actually have a story in our backlog about having some kind of release script that replaces all generic instructions with release specific ones when releasing. This would fit nicely into it.