Add OSSU website URL in the repo About section to increase discoverability

[ishriyaq16]

On Github, we are used to seeing the URL of docs/homepage of a project in the About section. But the OSSU website URL is hyperlinked in the README only. For a little bit better discoverability, let's add the URL to the repo About section.

[waciumawanjohi]

Good point. Done!