DOC: adding artefact retention policy to SPEC4 (#262)

This is to address #261 
I'm on the opinion that it was a mistake to include so much
implementation detail in the SPEC itself, but as the other specifics of
https://anaconda.org/scientific-python-nightly-wheels are in the docs,
we should list these policies, too.


Also note, that we may eventually change the actual numbers, e.g.
scientific-python/upload-nightly-action#35


cc @mattip

spec-0004/index.md

@@ -102,6 +102,21 @@ workflow" add the following lines as an additional step:
 
 Complete examples of how projects implement this in their CI setup are linked in the Notes section.
 
+#### Artifact cleanup-policy at https://anaconda.org/scientific-python-nightly-wheels
+
+To avoid hosting outdated development versions, as well as to clean up space, we do have a
+retention policy of:
+
+- Latest **N versions**
+- Artifacts newer than **M days**
+
+Any versions beyond these are automatically removed as part of a daily cron job. Projects may
+have reasons to request to be added to the list exempt from this automated cleanup, however in that
+case the responsibility of cleaning-up old, unused versions fall back on the individual project.
+
+Note: The actual values for `N` and `M` are defined and documented in the
+https://github.com/scientific-python/upload-nightly-action repository.
+
 #### Process for Adding New Projects
 
 The site admins are drawn from active members from the scientific Python community.