Document eglot config.

[csasarak]

I had to do some additional setup to get scala-ts-mode working with Eglot. I added a section to the readme describing what I did.

[KaranAhlawat]

Thanks for your contribution!
PS: I think this should be changed upstream in Eglot itself so people don't have to think about it, but till then this is a good start.

[csasarak]

I think you're right. I checked the emacs mailing list and could not find any references to it. It's pretty simple to just add the scala-ts-mode to the eglot source though.