Add meme faces and README improvements

Thanks for developing this fun app and also for having published it on F-Droid, @jasonycw! :+1:

Three things I'd like to see improved:

Add the awesome meme/rage faces from http://www.memes.at/ (and possibly other sites)
Make pictures in your README a bit smaller so that your repository is better browsable
Add the F-Drod picture and direct link to your app below the name MemeCreator on your README

Preview to what I would prefer it to look like:

MemeCreator

Code to copy and paste into your README (let me know if you prefer a pull request):

# MemeCreator
[![F-Droid](https://upload.wikimedia.org/wikipedia/commons/thumb/0/0d/Get_it_on_F-Droid.svg/320px-Get_it_on_F-Droid.svg.png)](https://f-droid.org/repository/browse/?fdid=cs4295.memecreator "Get MemeCreator on F-Droid!")

jasonycw / MemeCreator

Add meme faces and README improvements #11