Add note about testing whether a Java runtime is installed

Author: pletcher

README.md

@@ -8,6 +8,8 @@
 ## Instructions
 
 1. Install a recent Java runtime, if you haven't already. We recommend 1.8, but 1.7 will work, too.
+  1. If you've been following along on Learn, we've walked you through installing Java (both the JRE and the JDK).
+  2. But if you want to make sure, you can try running `java` or `javac` at your command line prompt. If it doesn't work, you probably don't have Java installed.
 2. In `com.flatironschool.javacs.Main`, we've provided a function `getVersion()`. It should print your system's current version of Java.
    1. We want not only to make this function work but also to make sure that it prints a version compatible with the upcoming lessons.
    2. You might find the following information useful: