Add CHANGELOG.md and API.md

[ademarco]

We should start keeping a CHENGELOG.md, we could base it on http://keepachangelog.com/en/1.0.0/ since we don't have resources to build an automatic changelog generation tool then we will start with a manually generated one.

We should also add an API.md which will define the API for the project, i.e. the contract we will bind our major release on. API policy is to be based on: https://symfony.com/doc/current/contributing/code/bc.html

[ademarco]

After discussion with team member we would go with a changelog generation in other language than php.