DOC: add channel retention policy (#38)

Author: bsipocz

README.md

@@ -4,7 +4,7 @@ This is a GitHub Action that uploads nightly builds to the [scientific-python ni
 as recommended in [SPEC4 — Using and Creating Nightly Wheels][].
 
 In a GitHub Actions workflow (`.github/workflows/*.yaml`), use the
-following snippet to upload built wheels to the repository:
+following snippet to upload built wheels to the channel:
 
 ```yml
 jobs:
@@ -36,19 +36,30 @@ updates:
       interval: "weekly"
 ```
 
-## Access to the ``scientific-python-nightly-wheels`` space
+## Access to the ``scientific-python-nightly-wheels`` channel
 
 To request access to the wheel channel, please open an issue on [the upload action's
 repository](https://github.com/scientific-python/upload-nightly-action). You can
 then generate a token at `https://anaconda.org/scientific-python-nightly-wheels/settings/access`
 with permissions to _Allow write access to the API site_ and _Allow uploads to Standard Python repositories_,
 and add the token as a secret to your GitHub repository.
 
+## Artifact cleanup-policy at the ``scientific-python-nightly-wheels`` channel
+
+To avoid hosting outdated development versions, as well as to clean up space, we do have a
+default retention policy of:
+
+- Latest **5 versions**
+- Artifacts newer than **30 days**
+
+Any versions beyond these are automatically removed as part of a daily cron job run from this repository.
+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.
 
 # Using nightly builds in CI
 
 To test against nightly builds, you can use the following command to install from
-the nightly repository:
+the nightly channel:
 
 ```sh
 python -m pip install \