doc: modernize README.md and add CoC

[aqrln]

• Remove obsolete and misleading information from README.md.
• Make README.md look cleaner.
• Add a Code of Conduct.
• Add a simple introductory example to the front page and duplicate it in examples/readme-example.
• Add the list of maintainers.

[belochub]

@aqrln, please add Refs: https://github.com/metarhia/JSTP/issues/163 to commit description.

[belochub]

@aqrln, this PR also addresses https://github.com/metarhia/JSTP/issues/157, this https://github.com/metarhia/JSTP/issues/157#issuecomment-300619954 in particular, but it is up to you whether to mention this in commit description or not.

[belochub]

@aqrln, additions of examples is connected to https://github.com/metarhia/JSTP/issues/131, you should also mention this in commit description.

[aqrln]

@belochub I've updated the commit message with appropriate references, thanks!

[aqrln]

@metarhia/jstp-core given the facts that this PR has already been reviewed by all the possible maintainers (so there's no one to object or request changes anymore) and this is a change we'd like to happen ASAP ('cause the current README is kinda misleading), is it okay for me to go ahead and land this without the minimal waiting time?

[lundibundi]

@aqrln I think we discussed that it can be landed right away it all maintainers positively reviewed the pr, ain't we?

[aqrln]

@lundibundi I don't recall us to have discussed that specific case, but it seems rather logical to me, so I'm gonna merge it right away. Please lmk if this is something that shouldn't have happened.

[aqrln]

Landed in f6a3e92c1eea2ba87f658e8618512c6efad048f1 🚀