Add readme to explain how to use gradle and setup gradle for newbies.

Author: shiling

USING_GRADLE.md

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+# Using Gradle to Build the project
+
+This project uses gradle to install external dependencies, build the project (jar files), and execute tests.
+
+## Gradle Version
+
+The version of gradle required is Gradle 4.6, or as specified in `/gradle/wrapper/gradle-wrapper.properties`.
+
+## Run tasks using gradlew instead of gradle
+
+Run `./gradlew` instead of the globally installed `gradle` on your machine to execute gradle tasks.
+
+This ensures that the correct version of gradle is used, and also downloads the required version of gradle.
+
+## Tasks
+
+### Build buildJavaCommonsCore
+
+Run `./gradlew buildJavaCommonsCore` to build the JavaCommonsCore module.
+
+### Build the jar packages
+
+Run `./gradlew jar` to build the project and package as a jar.
+
+Run `./gradlew shadowJar` to build the project and package as a shadow jar (dependencies are shaded)
+
+Run `./gradlew fatJar` to build the project and package as a fat jar (dependencies are included, and not shaded)
+
+Run `./gradlew buildAll` to build all 3 jars (jar, shadow jar, and fat jar).
+
+Run `./gradlew testAndBuildAll` to run full test suite and build all 3 jars (jar, shadow jar, and fat jar).
+
+## Setting  up the project in IntelliJ
+1. Right click "settings.gradle" and click "Import Gradle Project"
+2. In the Gradle Tool Window, click the wrench, and then "Gradle Settings"
+3. Check "Download external annotations for dependencies" - this will allow code hinting to work
+4. Set "Use Gradle from" to "'gradle-wrapper.properties' file" to make sure it runs the correct version of gradle
+
+