Improve documentation on usage

[davesnx]

I need to check the steps on this gist in order to publish a package properly.

Could this be part of the repo/documentation?

[gpetiot]

Steps 4, 5 and 6 from this gist are just dune-release, so basically:

• optional: integrate dune-release lint to your CI
• dune-release tag x.y.z
• dune-release

[davesnx]

For the happy path, sure dune-release works out of the box, but I haven't had a single run succeeding at it, and I found a few error states:

• bad github token (+ really bad error message and where to setup it)
• it didn't consider changelog
• failed at dirty repo
• failed when tag is already pushed
• missing git modules

So, the guide I'm thinking is to handle all of those steps and explain those quirks we have been facing into a guide