Add dependencies to readme instructions

README.md

@@ -15,6 +15,10 @@ to work well:
     unzip gradle-5.6.1-bin.zip
     export PATH=$PWD/gradle-5.6.1/bin:$PATH
 
+You will also need a `java` binary installed:
+
+    sudo apt install default-jre # or something equivalent
+
 Clone and compile:
 
     git clone https://git.hush.is/hush/SilentDragonWormhole