Updated README.md for Ruby [skip ci]

[kpumuk]

In a preparation for 0.23.0 release, let's update Ruby documentation page. Current one simply states:

This section should get written by someone with the time and inclination. In the meantime, look at existing code, such as the benchmark or the tutorial in the full thrift distribution.

New one explains a bit more (generated locally by https://github.com/apache/thrift-website):

• Did you create an Apache Jira ticket? (Request account here, not required for trivial changes)
• If a ticket exists: Does your pull request title follow the pattern "THRIFT-NNNN: describe my issue"?
• Did you squash your changes to a single commit? (not required, but preferred)
• Did you do your best to avoid breaking changes? If one was needed, did you label the Jira ticket with "Breaking-Change"?
• If your change does not involve any code, include [skip ci] anywhere in the commit message to free up build resources.

[kpumuk]

Sorry, forgot to add [skip ci]. Rebased to amend the commit message