Wanted To Add PWA support to g2ttrss-mobile

[GraysonPeddie]

Hi. My name is Grayson Peddie and I am brand new to pull requests. I want to add support for Progressive Web Applications (PWA) to g2ttrss-mobile and I don't know how to submit a pull request.

I did some research about how I can make web applications PWA-enabled so that mobile users can add shortcut to a home screen where there's a + symbol next to the address bar in Firefox for Android.
https://developer.mozilla.org/en-US/docs/Web/Progressive_web_apps/Installable_PWAs

Attached is a zip file containing three files: g2ttrss-mobile-pwa.webmanifest (which adds PWA support), index.htm (I added a manifest link), and README.md (which tells the user of g2ttrss-mobile to modify "start_url" in the g2ttrss-mobile-pwa.webmanifest file).

g2ttrss-mobile-pwa.zip

I hope I can be of use to others and I want to contribute to your project. It's working in Android although I do not have an iPhone to test.