Update the README instructions to clarify the importance of pulling submodules (firebase#1939)

* Initial commit * Feedback

Author: var-const

README.md

@@ -53,9 +53,12 @@ https://firebase.google.com.
 ## Getting Started
 
 * Install the latest Android Studio (should be 3.0.1 or later)
-* Clone the repo (`git clone git@github.com:firebase/firebase-android-sdk.git`)
+* Clone the repo (`git clone --recurse-submodules git@github.com:firebase/firebase-android-sdk.git`)
+ * When cloning the repo, it is important to get the submodules as well. If
+ you have already cloned the repo without the submodules, you can update the
+ submodules by running `git submodule update --init --recursive`.
 * Import the firebase-android-sdk gradle project into Android Studio using the
- **Import project(Gradle, Eclipse ADT, etc.** option.
+ **Import project(Gradle, Eclipse ADT, etc.)** option.
 
 ## Testing