Edit README

Add short description for project, how to run the project, and tested
environment.

Author: shinwan2

README.md

@@ -1,7 +1,19 @@
 PostMaker
 =========
 
-Simple social media application for Android.
+Simple app to post content for Android. This app utilizes Firebase as backend services, i.e.
+Firebase SDK Authentication and Firebase Realtime Database. Signed in users can post to a timeline,
+view posts, and delete his/her own post.
+
+### How to run
+1. You need to set up your own Firebase Project in [Firebase Console](https://console.firebase.google.com).
+    Spark plan is free with a certain limit.
+2. Once set up, download and place your `google-services.json` in `/app` directory.
+3. Build and run this project.
+
+### Tested Environment
+1. Android Device minimum API Level 19
+2. Built using Android Studio 3.2.1
 
 License
 =======