Guidelines for contributing

[akarnokd]

As called for by #3145, I'd like to collect the checkpoints one has to conform with when posting a PR:

Style

• Use 4 spaces instead of tabs
• Use single newline (not cr+lf)
• Otherwise, there aren't any concrete style requirements.

When changing existing content:

• Don't change existing unit tests unless you are fixing a bug in them.
• Don't rename existing classes, fields and local variables unless you are fixing a bug or doing (performance) improvements and even then, limit yourself to the target area.
• Generally try to avoid cosmetic or style changes.

When proposing new content:

• Take into account that accepting new public API extensions (i.e., operator on Observable or anything outside rx.internal) may take weeks to accept and several rounds of review.
• Make sure the proposed operator has sufficient amount of tests. If the operator is involved in concurrency or coordination, have looped unit tests that test for concurrent behavior for a certain number of rounds (but don't make it take minutes to complete).