[Meta] Add code of conduct and update contributor instructions

[robfrawley]

Q	A
Branch?	2.0
Bug fix?	no
New feature?	no
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets
License	MIT
Doc PR

This PR includes a standard copy of the Contributor Covenant Code of Coduct, updated to include an independent mailing list for abuse reports (imagine-bundle@googlegroups.com). This doesn't have to be the final end-point for the code of conduct email; please let me know if the project owners would prefer a different mailing list service or something else entirely. Otherwise, I can grant admin rights to that Google Group account to those who should have it.

Let me know if any of the project owners do not want to include a code of conduct, or would like modifications to it. It would be great if we could stick with the standard one, though.

@lsmith77 @lipp thoughts?

Additionally, I improved the contributing file and included a notice about the code of conduct in the readme.

[lsmith77]

Thanks you for this initiative!

I definitely want to see a CoC for all of the Liip OSS projects. One of the tricky bits there is however enforcement. What I am currently working on in the scope of the Symfony diversity initiative is to setup a CoC (also using the covenant) and an enforcement process along with an enforcement team (which will get trained). The goal is to then open up this infastructure to the entire Symfony eco-system via an opt-in process. Hopefully this will be done in March/April.

But I am fine adopting the CoC now with this very minimal enforcement process. But I am also fine waiting until the above mentioned process is put into place. As long as that doesn’t end up being delayed.

[robfrawley]

Let's merge this and then we can change our enforcement policy once there is an infrastructure setup.
I'd like for the CoC to be part of the upcoming 2.0 release. Sent you an invite to the google group.

[robfrawley]

@lsmith77 Also, can we change the 2.0 branch to the repository default?