Updated README to include firebase-crashlytics-ndk build requirements.

README.md

@@ -61,6 +61,9 @@ https://firebase.google.com.
     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.
+* `firebase-crashlytics-ndk` must be built with NDK 21. See
+  [firebase-crashlytics-ndk](firebase-crashlytics-ndk/README.md) for more 
+  details.
 
 ## Testing
 

firebase-crashlytics-ndk/README.md

@@ -11,9 +11,13 @@ Initialize them by running the following commands:
 
 ## Building
 
-All Gradle commands should be run from the root of this repository.
-
-`./gradlew :firebase-crashlytics-ndk:assemble`
+* `firebase-crashlytics-ndk` must be built with NDK 21. Use Android Studio's
+  SDK Manager to ensure you have the appropriate NDK version installed, and
+  edit `../local.properties` to specify which NDK version to use when building
+  this project. For example:
+  `ndk.dir=$USER_HOME/Library/Android/sdk/ndk/21.4.7075529`
+* All Gradle commands should be run from the root of this repository:
+  `./gradlew :firebase-crashlytics-ndk:assemble`
 
 ## Running Tests