Added some Java installation instructions.

Includes note about JAVA_HOME environment variable as per #81.

README.md

@@ -33,6 +33,11 @@ Development of JPhyloRef takes place in [our GitHub repository]. This includes [
 
 # Build and execution instructions
 
+You will need [Java] and [Apache Maven] to build the software from source. We recommend installing these
+tools using a package manager, such as [Homebrew] on macOS. You can use [jEnv] instead if you want to install
+multiple Java versions on the same computer. Installation should set up the `JAVA_HOME` environment
+variable; if not, you will need to set it to point at the directory containing your Java installation.
+
 Once you have downloaded the source code to your computer, you can compile and test the code by running `mvn test`.
 
 JPhyloRef can be built from source by running `mvn package` from the root directory of this repository. This will create a JAR file in the `target/` directory, which can be executed by running,
@@ -88,6 +93,10 @@ Once you're set up, you can run `mvn clean deploy` to publish the package
 to the OSSRH. If your version number ends in `-SNAPSHOT`, this will be
 published to the OSSRH Snapshots repository.
 
+  [Java]: https://www.java.com/en/
+  [Apache Maven]: https://maven.apache.org/
+  [Homebrew]: https://brew.sh/
+  [jEnv]: https://www.jenv.be/
   [Sonatype OSSRH]: https://central.sonatype.org/pages/ossrh-guide.html
   [the Sonatype website]: https://central.sonatype.org/pages/apache-maven.html
   [Maven settings.xml file]: https://central.sonatype.org/pages/apache-maven.html