Update README.md

README.md

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 # setup-java
 
-This action sets the java environment for use in actions
+This action sets up a java environment for use in actions by:
+
+- optionally downloading and caching a version of java by version and adding to PATH. Downloads from [Azul's Zulu distribution](http://static.azul.com/zulu/bin/).
+- registering problem matchers for error output
+
+# Usage
+
+See [action.yml](action.yml)
+
+Basic:
+```yaml
+actions:
+- uses: actions/setup-java@latest
+  with:
+    version: 9.0.4 // The JDK version to make available on the path. Use a whole version, such as 9.0.4, not a semver version
+    architecture: x64 // (x64 or x86) - defaults to x64
+- run: java -cp java HelloWorldApp
+```
+
+From local file:
+```yaml
+actions:
+- uses: actions/setup-java@latest
+  with:
+    version: 4.0.0
+    architecture: x64
+    jdkFile: <path to jdkFile> // Optional - jdkFile to install java from. Useful for versions not supported by Azul
+- run: java -cp java HelloWorldApp
+```
+
+Matrix Testing:
+```yaml
+jobs:
+  build:
+    strategy:
+      matrix:
+        java: [ 6.0.119, 9.0.4, 12.0.2 ]
+    name: Java ${{ matrix.java }} sample
+    actions:
+      - name: Setup java
+        uses: actions/setup-java@latest
+        with:
+          version: ${{ matrix.java }}
+          architecture: x64
+      - run: java -cp java HelloWorldApp
+```
+
+# License
+
+The scripts and documentation in this project are released under the [MIT License](LICENSE)
+
+# Contributions
+
+Contributions are welcome!  See [Contributor's Guide](docs/contributors.md)