Combine info on changes Conventions

[erget]

Some discussion with @JonathanGregory on minting 1.10 brought us to this issue. I've volunteered to look at this at the upcoming workshop and won't fill this with too much details, but here's some context so I'll remember what it's about:

Do you think that rules.md is the right place to document these? It sounds to me more like modalities of working with GitHub, in which case CONTRIBUTING.md might be a better home for these guidelines.

...These two documents [rules.md and CONTRIBUTING.md] overlap somewhat in purpose. Could we merge them? In order to do that, we would have to put the rules for making enhancements and rules for correcting defects into the same file, along with the contents of CONTRIBUTING.