Update OS X instructions

Getting-Started.md

@@ -133,10 +133,18 @@ Then, clone the repository to your local machine:
 
 > git clone https://github.com/micropython/micropython.git
 
+Change directory to the git repo and fetch all the submodules:
+
+>  git submodule update --init
+
 Change directory to the Unix build directory:
 
 > cd micropython/unix
 
+Make the dependencies:
+
+> make axtls
+
 And then make the executable
 
 > make