Add instructions to mojo README to cover installing build dependencies.

README.md

 Mojo
 ====
 
 This repo uses gclient to manage dependencies, so to build things from this
 repo you have to first download depot_tools and make sure it is in your path
 
 http://www.chromium.org/developers/how-tos/install-depot-tools
 
 then create a file called '.gclient' that has the following contents:
 
 ```
 solutions = [
   { "name"        : "src",
     "url"         : "https://github.com/domokit/mojo.git",
     "deps_file"   : "DEPS",
     "managed"     : False,
     "safesync_url": "",
   },
 ]
 # Include the following line only if you're interested in building for android.
 target_os = ['android']
 ```
 
 Then run 'gclient sync' from the directory containing the .gclient file.
 This will clone this repository into a subdirectory called 'src'.  From that
-directory, you can then build by running:
+directory, you can then ensure you have all the dependencies installed by
+running:
+
+```
+$ ./build/install-build-deps.sh
+```
+
+You can then build by running:
 
 ```
 $ gn gen out/Debug
 $ ninja -C out/Debug mojo
 ```