Improve README.md

Hi Peter,

I reviewed my older open source commitments and therefore stumbled upon your GitHub repository again. 

First of, compliments for keeping APK Downloader uptodate! :+1: 

Now, I noticed that you still use [the icon that I created for my version](https://github.com/eyecatchup/android-play-store-apk-downloader-crx/commit/c3985ac757dcb6c10bf053590401f824490e96a5). Which is perfectly fine, but just the credit for it was missing. So I wanted to update the README. Then I noticed that you seem to have missed my repository (Yep, there was already a public git repositry before ;)). So while I was on it, I also updated and reformatted some other parts.

If you're fine with the changes, I'd appreciate it if you'll merge this change. Otherwise, please let me know what's wrong with it. Thanks in advance! :)

Cheers!