assert/CONTRIBUTING.md on master

[ktomk]

Wanted to file a PR for reporting an issue and followed How to contribute.

Thanks for the nice introduction in it and also for not being extensive.

But already with the second bullet-point:

• Coding standard compliance must be ensured before committing or opening pull requests by running composer assert:cs-fix or composer assert:cs-lint in the root directory of this repository.

I run into an issue as when I execute it on a clean checkout of master, it already modifies files. And some of the fixes even don't look good to me. So just wondering if they (the composer script commands) currently apply and how to proceed. Unit tests run so I hope this ain't a blocker. Maybe it's just me or the tooling.

If interested, I've put diffs and docs into a fake PR on my fork, as both the diffs and the typescript is verbose: ktomk#1