README updates

[hashhar]

Most of this is "refactor".

Actual useful/impacting content changes are:

• Remove SQLAlchemy pre-requisite as setup.py will already install it
• Simplify and reorder development instructions in README
• Update releasing instructions in README

PTAL @ebyhr at Update releasing instructions in README specially. I'm not sure if the trailing slash was intentionally left out (to prevent copy and paste?).

[hashhar]

I left out pre-commit setup for now since CI enforces things for now but we can decide what parts to offload to PR authors (e.g. import sorting, automated reformat).

I'm wary of requiring something like mypy enforced on PR authors (it must be enforced in CI however) because not everyone is familiar with it and we want people to be able to open PRs and we can help with parts they aren't confident about. We can raise the bar over time.

[hashhar]

Thanks!. Re I left out pre-commit setup for now are you referring to #208 (comment) ?

Yes, I mean someone should add pre-commit setup instruction to README. I also want to look into whether it's possible to have "profiles" in pre-commit so that CI is more strict than local.

Or alternatively we can only add things we really want to check before commit to pre-commit (like sort imports, auto reformat) but keep other things like mypy and tests outside of it.

[hashhar]

I'll follow up with dedicated DEVELOPMENT.md/CONTRIBUTING.md.