Create github-administration.md

Explain the tags that we plan to use to organize repositories in Plone GitHub organization

Author: gforcada

docs/contributing/github-administration.md

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+# GitHub administration
+
+## Repository tagging
+
+To easily find repositories and what's their purpose within the Plone organization,
+a set of tags are being set on repositories within the Plone GitHub organization.
+
+The tags are so far:
+
+- `core`: any repository that are installed when doing a volto or a classic ui project, i.e. what's shared between the two
+- `volto`: specific repositories that are meant to get volto running
+- `volto add-ons`: repositories not part of default volto UI/UX but meant for it
+- `classic ui`: counterpart to volto tag
+- `classic ui add-ons`: counterpart to volto add-ons
+- `documentation`: repositories that have documentation
+- `training`: repositories related to the training documentation
+- `infrastructure`: repositories used for all about maintaining Plone servers
+- `tooling`: tools / apps needed to orchestrate Plone