docs: Update README to match OpenFeature template

[maxveldink]

This PR

• adds a basic CONTRIBUTING.md file
• updates README.md to match OF template and be parseable by release-please

Related Issues

Closes #120 Closes #45

Follow-up Tasks

• Once this is released, will open a PR to have the ruby-sdk listed on the OF website

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.54%. Comparing base (e4dcb04) to head (9e155c2).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #147 +/- ## ======================================= Coverage 99.54% 99.54% ======================================= Files 16 16 Lines 218 218 ======================================= Hits 217 217 Misses 1 1 ```

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