Add note about TypeScript's implementation

[jaller94]

An experimental implementation was added to TypeScript in microsoft/TypeScript@62ba369.
One must enable the compiler flag experimentalDecorators to use it.

[trotyl]

What implemented in TypeScript is not the decorator proposal in this repo, but a different proposal known as stage-0/stage-1 decorator.

[jaller94]

Feel free to close this then.
My goal was to raise the working group's awareness of this implementation, as the TypeScript documentation links to this repo and mentions the stage-2 proposal.
https://www.typescriptlang.org/docs/handbook/decorators.html

[trotyl]

See compat-table/compat-table#1047 for more information, mentioning of decorator implementation in TypeScript is wrong should be completely removed.

[littledan]

I think it's useful to have this context and explain how it's a different proposal. Let's write that up on top of #250 , which I expect to land soon.

[littledan]

Closing this PR, as the current explainer documents the relationship between TypeScript decorators and these.