Update README.md to correct broken donate images

[DanDawson]

The images you linked to in your Dropbox account do not display on the README.md file. To generate a URL for embedding in Dropbox you have to use their Share button to generate a sharable URL. Since a non-owner can't do that, I went ahead and found the current Patreon logo (they changed logos since you linked it) and used that URL instead.

I also added an image link to the same style of donate "tip jar" image you had used.

I went ahead and manually set them to 100px height (you can adjust as needed), and since GitHub's Markdown doesn't allow image sizing I used HTML.