Add documentation explaining how to install the PWA if onBeforeInstall() event is not supported by the browser

[Jaifroid]

As an outcome of #625, I think I should add some documentation explaining how to install / add to home screen the PWA in browsers that don't support the install event. Although all Chromium browsers should support it, in some cases (e.g. Vivaldi) the function appears to be supressed for security reasons, and in others, like Firefox on Android or Safari on iOS, it is not supported even though the app can in fact be added to Home and can work as an independent app without any browser chrome.