Add contribution guidelines

[kachar]

Setting guidelines for repository contributors

• https://docs.github.com/en/free-pro-team@latest/github/building-a-strong-community/setting-guidelines-for-repository-contributors

Motivation

Have clear process (workflow) for contributions

Samples:

• https://github.com/atom/atom/blob/master/CONTRIBUTING.md
• https://github.com/rails/rails/blob/master/CONTRIBUTING.md
• https://github.com/opengovernment/opengovernment/blob/master/CONTRIBUTING.md

[kachar]

Added Branching strategy and Translation guidelines