Update the text about Firefox not supporting WebMIDI

I'm now using Pianojacques on Firefox successfully When WebMIDI is not available, the redirection should not be done to a firefox.html page saying that Firefox doesn't support WebMIDI anymore.

I've run into the redirection on both Chrome and Firefox by opening index.html directly, or by accessing it on 0.0.0.0.

The redirection should probably be done to a webmidi.html page telling users to:

• check that their browser is up to date and correctly configured (with an advise to use Chrome or Firefox; apparently Safari doesn't support WebMIDI)
• check that Pianojacques is accessed from HTTPS, or from a local http server (using localhost - 0.0.0.0 won't work)

The README should also probably advise developers to run a local http server using something like python3 -m http.server 8000 or php -S 127.0.0.1:8080 and access it using localhost:8000 or 127.0.0.1:800 (and not 0.0.0.0)