Added CODE_OF_CONDUCT.md

[parthrc]

Added CODE_OF_CONDUCT file based on Contributor Covenant for better contributor experience.

[michalmuskala]

I'm not against a code of conduct, but this one seems excessive for a project with a single maintainer and a handful of contributors that gets a new release once a year.

The language talking about "community leaders" just makes no sense whatsoever, when it's just me. Similarly "community spaces" doesn't make much sense for a project that is just here, on gihtub and doesn't have any other channels. TBH I don't really intend to build a community around this project - this is just a JSON library.

I'd be more comfortable adapting Elixir's or Phoenix's code of conduct, both of which are based on earlier versions of Contributor Covenant, seem much simpler & to the point.

[parthrc]

Hey @michalmuskala that makes sense, let me know if I should work on either of those you mentioned or this is not something you need at the moment. Cheers.