Update README to reflect our changes

[wusyong]

We have forked cargo-mobile and maintain this for a while. But we never update the README so users might get confuse on where to start.
We should add this note to README. The note itself is also outdated a bit.
Besides, we should also update wry's README to redirect mobile instructions to here.

[Adimac93]

I agree, README should be updated.

[jdc-cunningham]

Hmm the info on that hackmd link is definitely a lot different than the readme on this repo ha 😅

I will try to run this again on Win 10, if it works I'll volunteer to do update the readme if it's literally just copy paste from hackmd into this repo.

Edit: as a data point I tried to use the current readme on this repo and my project didn't fully run. I also had to enable developer mode on Windows which I thought was odd as I haven't had to do that before.