TLA+ coding conventions

# Summary

Problem: we don't have any conventions or standards on writing TLA+ code. A quick web search does not reveal any existing resource either.

We should try to define a minimal set of conventions & examples for helping us write TLA+ code consistently.

# Motivation & description

@ancazamfir recently raised the issue of inconsistent coding style in our TLA+ specs. There's already quite a few of us writing TLA+. Among others, cc-ing here: @josef-widder @konnov @istoilkovska @milosevic @ancazamfir. Without a pre-existing standard in the community, and without a basic guideline among ourselves, it is inevitable that each of us will develop (or stick to) their own style. But this would not be productive, nor will it help us in writing quality code.

It would be useful to:
- have a consistent writing style among ourselves.
- be simple and understandable, both for insiders and outsiders.