Add to Readme: how to use external tsc

[dpoetzsch]

It took me quite a while now to figure out you use a bundled tsc by default (with version still at 1.6).

After some searching I found a very easy flag to use an external (more recent) tsc, using the following line in a typescript.rb initializes:

TypeScript::Src.use_external_tsc = true

Maybe it would help others to add this option to the readme as it allows for so many up-to-date features? Especially, @types installed via yarn just work then ;-)

[cdimartino]

This was invaluable. Thank you!

[G-Rath]

@dpoetzsch that doesn't seem to be working for me - I've got the latest typescript, and am doing

const myFunc = <T extends object>(param: T) => ({ ...param });

This compiles fine, but ruby gives error TS2698: Spread types may only be created from object types..

This was a feature added in 3.2. Any advice?