README Polishing

This change polishes up the README by adding a note about initializing the
dropsonde submodule.

Author: nebhale

README.md

@@ -290,6 +290,7 @@ API Documentation for each module can be found at the following locations:
 The project depends on Java 8.  To build from source and install to your local Maven cache, run the following:
 
 ```shell
+$ git submodule update --init --recursive
 $ ./mvnw clean install
 ```