As we discussed on the team meeting we should make the life of new
contributors easier.
One easy thing is to make the contributor guide more visible by linking
it from README.rst not just from CONTRIBUTING.rst.

[1]: https://meetings.opendev.org/meetings/networking/2022/networking.2022-07-05-14.00.log.html#l-56

Change-Id: I6c211ed6c4fb8efb3230ab6c48fc1039738a5a15