Update Readme

[Luraktinus]

godot 3 is out, now it is "port or die"

[TeddyDD]

Thanks for PR :)

I'm working on 3.0 port here https://github.com/TeddyDD/gterm/tree/godot3porting
I'm going to merge it into master when I fix few bugs (after passing the semester).
If you want you can send this PR again targeting godot3porting branch, don't mention 2.1.x there.

https://github.com/TeddyDD/gterm/issues?q=is%3Aissue+is%3Aopen+label%3A3.0

[Luraktinus]

nice, i am new but i try to help

[TeddyDD]

Cool! If you want I can tell you how you can help:

1. Modify README according to Update Readme at godot3porting brach #6 (change required Godot version to 3.0 and link godot2 branch as last 2.1.x version)
2. Open new PR but choose godot3porting branch as target
3. Boom Update Readme at godot3porting brach #6 is solved :)

If you have any questions feel free to ask! I'll be quite busy for week or two. I can't work on gterm right now, so any help is welcome.