Add publishing instruction in contributing.md

[frinyvonnick]

I just published a version using lerna version since I can't figure out how to use correctly lerna publish.

Firstly I used the command lerna version premajor to bump an alpha version in every packages. Then I used the command yarn publish --tag alpha and type the current version in the prompt in each package to publish this version to npm.

After this, I had to execute gitmoji-changelog and amend release commit and move the git tag and push force it on master.