Update CONTRIBUTING.md

CONTRIBUTING.md

 
 ```
 curl -o- -L https://yarnpkg.com/install.sh | bash
+export PATH=$PATH:$HOME/.yarn/bin
+```
+
+* Android SDK:
+
+```
+export ANDROID_HOME=$HOME/android-sdk-macosx
+export PATH=$PATH:$ANDROID_HOME/tools:$ANDROID_HOME/platform-tools
+
+# fix for https://code.google.com/p/android/issues/detail?id=223424
+mkdir -p ~/.android
+
+# download android SDK
+echo "Downloading Android SDK"
+curl --location https://dl.google.com/android/android-sdk_r24.4.1-macosx.zip | tar -x -z -C $HOME
 ```
 
-* Android SDK
 * fbsimctl:
 
 ```
+export CODE_SIGNING_REQUIRED=NO
 brew tap facebook/fb && brew install fbsimctl
 ```