Add contribution guide

[MarkusH]

Summary of the changes / Why this is an improvement

Refs #63

Checklist

• CLA is signed

[mfussenegger]

lgtm

Although I do wonder if we should centralize the parts that are not necessarily project specific, like the git commit hygiene stuff. I think @infoverload also brought that topic up before?

[matthijskrul]

Similar from me - looks good in principle, but maybe the Git stuff ought to be located somewhere more obvious. (Though no harm in reminders, either.) Is there anything Kubernetes specific that needs adding? That's something a dev can better judge

[christianbader]

@matthijskrul @mfussenegger Agree, but I think almost non of the text is necessarily project specific and it's almost ident with the CrateDB contribution guide. Would it then make sense to have one central contribution guide in our docs and just link to it and mention the project specific parts like debug instructions in the individual repository?

[christianbader]

We do this already for other repositories:
https://github.com/crate/croud/blob/master/CONTRIBUTING.rst

[mfussenegger]

See also recent edits made by @infoverload in crate/crate#10523

[MarkusH]

Thanks for your input. How would you like to move forward? @infoverload do you have a preferred way?

[MarkusH]

I briefly talked to @infoverload and we agreed to -- while aiming for a centralized contribution guide -- merging the PR for now to not have this block Hacktoberfest.