add publiccode.yml to repo

[publiccode.yml](https://yml.publiccode.tools/) seems some metadata standard for source code repositories for software used in public administration
the file lives at the source of a relevant repository
for pycsw maybe the file can be generated from other sources in a ci-cd process?
this file seems used by repositories which catalogue software for public administration use like https://oss.developer.overheid.nl/repositories
an example of such a file is at https://github.com/Geonovum/ogc-checker/blob/main/publiccode.yml

Start with the publiccode.yml specification and the linked Geonovum example, then review pycsw's repository metadata and documentation for the values to publish. Add publiccode.yml at the repository root with pycsw's metadata, and verify whether the catalog accepts it; clarify whether CI generation is needed before implementation.

Written by the indexing model from the issue text.