Say the Gradle plugin works with the configuration cache

formatDiff and the Spotless step failed with Gradle's configuration
cache on (openjavaformat/open-java-format#193). The Gradle page now
says the plugin's tasks work with it, which versions up to 21.0.1 do
not, and that Gradle's own idea task turns it off.

To merge with the release that has the fix: until then the page would
promise what the published plugin does not do.

Author: abashev

docs/get-started/gradle.md

@@ -115,6 +115,22 @@ is resolved by the root project, so the root project needs the repository too.
     }
     ```
 
+## Configuration cache
+
+The plugin works with Gradle's configuration cache: `formatDiff`, the [Spotless](spotless.md) step
+and the IntelliJ IDEA tasks below run from a cached configuration, and the next build reuses it.
+Turn the cache on as for any build:
+
+``` properties title="gradle.properties"
+org.gradle.configuration-cache=true
+```
+
+Up to 21.0.1, `formatDiff` and the Spotless step fail with the cache on
+([#193](https://github.com/openjavaformat/open-java-format/issues/193)).
+
+Gradle's own `idea` task does not support the cache, and Gradle turns it off for a build that runs
+the task.
+
 ## IntelliJ IDEA
 
 When the project is opened in IntelliJ IDEA, the plugin writes the formatter settings into `.idea`,