Update README

Author: joecorall

README.md

@@ -525,6 +525,9 @@ Many dependencies in this repo are automatically updated using [renovate](https:
   - these dependencies are automatically merged when our CI tests pass using [automerge](https://docs.renovatebot.com/key-concepts/automerge/#automerging-and-scheduling)
 - [github-releases](https://docs.renovatebot.com/modules/datasource/github-releases/) and [github-tags](https://docs.renovatebot.com/modules/datasource/github-tags/) for software we install manually
 - [git-refs](https://docs.renovatebot.com/modules/datasource/git-refs/) when we pin to a specific commit on a branch
+- [apache-downloads](https://docs.renovatebot.com/modules/datasource/custom) a custom data source that parses the HTML list of releases ([e.g. solr/solr](https://downloads.apache.org/solr/solr/). This was done since using apache's GitHub repos is not a good indicator when apache cuts a release for their software. GitHub often has tags before apache officially releases their software, resulting in pre-emptive PRs from renovate before the release is available.
+
+##### Why isle-buildkit can't use Renovate's GitHub app to track updates
 
 Since renovate does not natively support the ability to extract a sha256 from a file, we need [a custom shell script](./ci/update-sha.sh) in the [postUpgradeTasks](https://docs.renovatebot.com/configuration-options/#postupgradetasks) to calculate the sha256 of our files and update our Dockerfile accordingly.