Add README.md file to the rhel tests/ruby repository to manage the content of how to contribute.

Only write rhel repository specific things referring to the Fedora tests/ruby README.md <https://src.fedoraproject.org/tests/ruby/blob/main/f/README.md>.

The contents can be:

The `tmt` command to run the tests via -h minute (1minuteTIP):

```
$ tmt -c distro=rhel-8 run --all provision -h minute -i 1MT-RHEL-8.6.0-released tests -n '^/Security/CVE-2017-0900-rubygems-No-size-limit-in-summary-length-of-gem/ruby27$'
```

The `tmt` command to run the tests via -h virtual. This is more complecated. But if you want to run the `-h vertual` really.

```
$ tmt -c distro=rhel-8 run --all provision -h virtual -i https://liver3.brq.redhat.com/1mt-vagrant/1MT-RHEL-8.6.0-released.box tests -n '^/Security/CVE-2017-0899-rubygems-Escape-sequence-in-the-summary-field/ruby27$'
```

Noting that Always use merge-requests rather than pushing commits directory to avoid breaking OSCI tests unintentionally.