Update & improve README

[gmazzap]

• Updated the list of rules in use (both imported and custom)
• Improved/less verbose wording
• Moved contribution guidelines to CONTRIBUTING.md

Please check if the PR fulfills these requirements

• [x] The commit message follows our guidelines
• [ ] Tests for the changes have been added (for bug fixes/features)
• [x] Docs have been added/updated (for bug fixes/features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

docs update

What is the current behavior? (You can also link to an open issue here)

n/a

What is the new behavior (if this is a feature change)?

n/a

Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

No

Other information: