Add OSX build instruction

Author: guillaumefe

README.md

@@ -1,2 +1,42 @@
 # APPetit
 Final version of the app
+
+### Installation on OSX
+
+**Install Android-SDK**
+`brew cask install android-sdk`
+
+**Export ANDROID_HOME**
+`export ANDROID_HOME=/usr/local/Caskroom/android-sdk/{android_sdk version number}`
+
+**Install OpenJdk8**
+`brew cask install adoptopenjdk/openjdk/adoptopenjdk8` 
+
+**Ensure OpenJdk8 is used**
+`export JAVA_HOME=/Library/Java/JavaVirtualMachines/adoptopenjdk-8.jdk/Contents/Home`
+
+**Accept Androind-SDK Licence**
+```
+cd /usr/local/Caskroom/android-sdk/{android_sdk version number}/tools/bin
+yes | sdkmanager --licenses && sdkmanager --update
+```
+
+**Get the Google Services Json**
+  - Sign in to Firebase, then create your project
+  - Add a name for your project and select the appropriate settings, then click on Create project.
+  - Click Settings icon, then select Project settings
+  - In the `Your apps` card, select the platform for the app you want created.
+  - Follow the required steps to add Firebase to your Android app. Here are the steps:
+    - Step 1 - Register app
+      - Package name : `com.mad.customer`
+      - Create local keystore file from command line : `keytool -genkey -v -keystore ~/.android/debug.keystore -alias androiddebugkey -keyalg RSA -keysize 2048 -validity 10000`
+      - Get the local debug certificate fingerprint : `keytool -list -v -alias androiddebugkey -keystore ~/.android/debug.keystore`
+    - Step 2 - Download config file `google-services.json`
+    - Step 3 - Move `google-services.json` to `{APPetit root folder}/customer/`
+    - Step 4 - Edit `{APPetit root folder}/build.gradle`, change `classpath` to `com.google.gms:google-services:4.3.3` (or whatever google tells you)
+
+**Build the app**
+```
+cd {APPetit root folder}/customer/
+gradle build  # apk is now located in {APPetit root folder}/customer/outputs/build/apk/debug and {APPetit root folder}/customer/outputs/build/apk/release
+```