Add upgrade documentation according to the charter

Per the charter

Need to add /docs/DEVELOPMENT_MAINTENANCE.md

With the following contents:

1. The process for upgrading
2. How to test the upgrade
3. Documentation for changes made to the upstream chart

Examples:
https://repo1.dso.mil/platform-one/big-bang/apps/developer-tools/gitlab/-/blob/main/docs/DEVELOPMENT_MAINTENANCE.md

https://repo1.dso.mil/platform-one/big-bang/apps/security-tools/keycloak/-/blob/main/docs/PACKAGE_UPDATES.md

https://repo1.dso.mil/platform-one/big-bang/apps/collaboration-tools/mattermost/-/blob/main/docs/PACKAGE_UPDATES.md