Updates the sbt cache docs in the README (actions#327)

Author: adamnfish

README.md

@@ -70,7 +70,7 @@ Currently, the following distributions are supported:
 The action has a built-in functionality for caching and restoring dependencies. It uses [actions/cache](https://github.com/actions/cache) under hood for caching dependencies but requires less configuration settings. Supported package managers are gradle, maven and sbt. The format of the used cache key is `setup-java-${{ platform }}-${{ packageManager }}-${{ fileHash }}`, where the hash is based on the following files:
 - gradle: `**/*.gradle*`, `**/gradle-wrapper.properties`
 - maven: `**/pom.xml`
-- sbt:  `**/build.sbt`
+- sbt: all sbt build definition files `**/*.sbt`, `**/project/build.properties`, `**/project/**.{scala,sbt}`
 
 The workflow output `cache-hit` is set to indicate if an exact match was found for the key [as actions/cache does](https://github.com/actions/cache/tree/main#outputs).