Add gradle command line instructions to build application

[alefq]

In order to facilitate beta-testing or continuous integration, app should be compilable through command line.
Instructions on how to do that should be added to Readme.
Example:

export ANDROID_HOME=/path/to/android/sdk

git clone https://github.com/RocketChat/Rocket.Chat.Android.git
cd Rocket.Chat.Android

echo "sdk.dir="$ANDROID_HOME > local.properties

./gradlew assembleDebug