Update rds_exporter branching and guideline

1. main as default and up to date branch [1]

2. clear contribution guideline according to this template.

3. PR template if needed and helpful

4. in doc guideline and/or PR template pmm-review-team should be mentioned as default assignee for the review.

See mongodb_exporter as an example.

Hints:

1. rename default branch to the main in github interface, so open PR would be also updated with it. For ppl's local branches, they need to update it, see mongodb_exporters old PRs for the example on how to.