Add additional getting started info to README

[addisonElliott]

Include details of how to install SDK into Android project using Gradle as well as compiling an AAR file to manually include into Gradle.

Add information from "Getting Started' section in docs:
http://docs.parseplatform.org/android/guide/#getting-started

This contains information on how to initialize Parse using the Manifest or custom Application class.

I included an additional step to the README which I believe is missing from the Getting Started docs. It explains that you must remember to reference the custom Application class in AndroidManifest.xml. See parse-community/docs#446

Fixes issue #705

[codecov]

Codecov Report

Merging #706 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #706   +/-   ##
=========================================
  Coverage     53.06%   53.06%           
  Complexity     1732     1732           
=========================================
  Files           132      132           
  Lines         10254    10254           
  Branches       1432     1432           
=========================================
  Hits           5441     5441           
  Misses         4364     4364           
  Partials        449      449

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[rogerhu]

Thanks for the updates!