Add Caching to GitHub actions and update README.md to reflect Gradle

.github/workflows/build.yml

@@ -18,6 +18,7 @@ jobs:
       with:
         java-version: 11
         distribution: 'adopt'
+        cache: gradle
 
     - name: Build with Gradle
       run: gradlew build

README.md

@@ -21,9 +21,9 @@ Now it's time to create your very own Multiverse server, do check out our [Wiki]
 
 Building
 ========
-Simply build the source with maven:
+Simply build the source with Gradle:
 ```
-$ mvn install
+./gradlew build
 ```
 More details are available on the [build instructions wiki page](https://github.com/Multiverse/Multiverse-Core/wiki/Building).