Add basic build steps to README.md (#185)

Summary:
For those not familiar with android and iOS development practices, adding basic build instructions.
Pull Request resolved: https://github.com/facebook/Sonar/pull/185

Reviewed By: passy

Differential Revision: D8991468

Pulled By: jknoxville

fbshipit-source-id: 990dc1217c7b5275d143b915c1b4872b271f4c94

README.md

@@ -41,7 +41,10 @@ Please refer to our [Getting Started guide](https://fbsonar.com/docs/getting-sta
 * iOS developer tools (for developing iOS plugins)
 * Android SDK and adb
 
-## Starting the desktop app
+# Building from Source
+
+## Desktop
+### Running from source
 
 ```
 git clone https://github.com/facebook/Sonar.git
@@ -52,11 +55,26 @@ yarn start
 
 NOTE: If you're on Windows, you need to use Yarn 1.5.1 until [this issue](https://github.com/yarnpkg/yarn/issues/6048) is resolved.
 
-## Building the desktop app
+### Building standalone application
 
 ```
 yarn build --mac --version $buildNumber
 ```
+## iOS SDK + Sample App
+
+```
+cd iOS/Sample
+pod install
+open Sample.xcworkspace
+<Run app from xcode>
+```
+
+## Android SDK + Sample app
+
+Start up an android emulator and run the following in the project root:
+```
+./gradlew :sample:installDebug
+```
 
 ## Documentation