Add Obtainium link in readme page

[pyhofficial]

Obtainium allows you to install and update apps directly from their release pages, and receive notifications when new releases are made available.

the link

Its markdown code

[![Get it on Obtainium](https://raw.githubusercontent.com/ImranR98/Obtainium/main/assets/graphics/badge_obtainium.png)](http://apps.obtainium.imranr.dev/redirect.html?r=obtainium://add/https://github.com/fork-maintainers/iceraven-browser/releases)
It's a alternative to #26 and #78