document the correct build procedure (#1675)

* CONTIBUTING.md:  correct build command, document its prereqs

* CONTRIBUTING.md:  expand the build instructions

CONTRIBUTING.md

@@ -111,10 +111,44 @@ Developers in the Contributors team can push directly to Main instead of submitt
 - In general, library changes that plausibly might break existing apps appear only in major releases, not minor ones.
 
 
-## Building the engine
+## How to build the Engine from source
 
-1. Install [Gradle](http://www.gradle.org/)
-2. Navigate to the project directory and run 'gradle build' from command line to build the engine.
+### Prerequisites
+
+These steps need only be done once...
+
+1. Install a Java Development Kit (JDK), if you don't already have one.
+2. Set the JAVA_HOME environment variable:
+  + using Bash: `export JAVA_HOME="` *path to your JDK* `"`
+  + using PowerShell: `$env:JAVA_HOME = '` *path to your JDK* `'`
+  + using Windows Command Prompt: `set JAVA_HOME="` *path to your JDK* `"`
+  + Tip: The path names a directory containing "bin" and "lib" subdirectories.
+    On Linux it might be something like "/usr/lib/jvm/java-17-openjdk-amd64"
+  + Tip: You may be able to skip this step
+    if the JDK binaries are in your system path.
+3. Clone the project repository from GitHub:
+  + `git clone https://github.com/jmonkeyengine/jmonkeyengine.git`
+  + `cd jmonkeyengine`
+  + As of 2021, this step consumes about 1.3 GBytes of filesystem storage.
+
+### Build command
+
+Run the Gradle wrapper:
++ using Bash or PowerShell: `./gradlew build`
++ using Windows Command Prompt: `.\gradlew build`
+
+After a successful build,
+snapshot jars will be found in the "*/build/libs" subfolders.
+
+### Related Gradle tasks
+
+You can install the Maven artifacts to your local repository:
+ + using Bash or PowerShell:  `./gradlew install`
+ + using Windows Command Prompt:  `.\gradlew install`
+
+You can restore the project to a pristine state:
+ + using Bash or PowerShell: `./gradlew clean`
+ + using Windows Command Prompt: `.\gradlew clean`
 
 ## Best Practices