Update CONTRIBUTING.md - fix #20 by adding a section on version upgrades

Signed-off-by: Jesse Hitch <jessebot@linux.com>

Author: jessebot

CONTRIBUTING.md

@@ -19,6 +19,10 @@ Then, please make sure you follow the [pull request template](.github/pull_reque
 
 ![screenshot of the allow edits by maintainers check box to the left of the Create pull request button on GitHub](https://github.com/nextcloud/helm/assets/2389292/3a8044a9-583d-496a-b3d2-4dd699c56ed4)
 
+## Upgrades
+
+If upgrading a subchart, such as PostgreSQL or MariaDB, or upgrading the `appVersion` in `./charts/nextcloud/Chart.yaml`, please never upgrade more than one major version at a time. Please also  provide documentation in the PR for how users can upgrade an external service locally.
+
 
 # Issues