adding contributing guidelines

| Q             | A
| ------------- | ---
| Bug fix?      | [no]
| New feature?  | [yes]
| BC breaks?    | [no]
| Deprecations? | [no]
| Tests pass?   | [yes]
| Fixed tickets | -
| License       | MIT
| Doc PR        | https://github.com/symfony-cmf/symfony-cmf-docs/commit/d63e63dcd6f0364d76e1b189bc0db91552498b13 (that should be a PR and not a commit normally, of course)

as discussed in the weekly irc meeting, we want to follow the symfony contribution guidelines. adding this file will make github show a link to the guidelines when you create a PR.

lets agree on the exact wording and all here, then we need to add that file to every repo that should follow the guidelines. that would be 

* every repo in the symfony-cmf organisation
* doctrine phpcr bundle
* sonata phpcr admin
* any others? i guess for jackalope / phpcr it makes less sense