Document git submodule update in COMPILING.md

Author: John Nonweiler

COMPILING.md

@@ -1,3 +1,11 @@
+# BEFORE COMPILATION
+
+JBMC uses a git submodule. This submodule must be set up before
+attempting a build:
+```
+git submodule update --init --recursive
+```
+
 # WHAT ARCHITECTURE?
 
 CPROVER now needs a C++11 compliant compiler and works in the following