Add ReadTheDocs configuration file

[jdvorak001]

The ReadTheDocs project page of the Guidelines tells us that:

Your project(s) "openaire-guidelines-for-cris-managers" don't have a configuration file. Configuration files will soon be required by projects, and will no longer be optional. Read our blog post to create one and ensure your project continues building successfully.

[ACz-UniBi]

Thank you @jdvorak001 for highlighting this.

How to add a configuration file for ReadTheDocs.

[jdvorak001]

The file seems to work, no more complaints from ReadTheDocs. Thank you @ACz-UniBi!