Add documentation for PLANTUML_ENCODING

doc/administration/integration/plantuml.md

@@ -115,6 +115,19 @@ that, login with an Admin account and do following:
 - Check **Enable PlantUML** checkbox.
 - Set the PlantUML instance as `https://gitlab.example.com/-/plantuml/`.
 
+NOTE: **Note:** If you are using a PlantUML server running v1.2020.9 and
+above (for example, [plantuml.com](https://plantuml.com)), set the `PLANTUML_ENCODING`
+environment variable to enable the `deflate` compression. On Omnibus,
+this can be done set in `/etc/gitlab.rb`:
+
+```ruby
+gitlab_rails['env'] = { 'PLANTUML_ENCODING' => 'deflate' }
+```
+
+From GitLab 13.1 and later, PlantUML integration now
+[requires a header prefix in the URL](https://github.com/plantuml/plantuml/issues/117#issuecomment-6235450160)
+to distinguish different encoding types.
+
 ## Creating Diagrams
 
 With PlantUML integration enabled and configured, we can start adding diagrams to