Creating onboarding, branch naming and standard convention guideline

[louisgv]

New contributor will need:

• Up-to-date Onboarding document (The current one might be deprecated)
• Branch naming convention, i.e @<github_username>/<feature-or-bug-name>
• Code convention with eslint and have a general guideline of having linter running before push. (We can enforce this with lint-staged).

[ColdSauce]

The Branch naming convention, i.e @<github_username>/<feature-or-bug-name> doesn't apply to contributors other than us since they won't have write access to our project. They would be working from their own pull requests.

[ColdSauce]

lint-stage with lerna is something I'll set up in another PR. Let's stick with just documentation here.