docs: regenerator-runtime dependency for swc (#1508) (#1509)

* docs: regenerator-runtime dependency for swc (#1508)

* docs(transpilers): update advice for installing regenerator-runtime

* Update transpilers.md

Co-authored-by: Andrew Bradley <cspotcode@gmail.com>

Author: petercoulton

website/docs/transpilers.md

@@ -20,10 +20,10 @@ We have bundled an experimental `swc` integration.
 [`swc`](https://swc.rs) is a TypeScript-compatible transpiler implemented in Rust.  This makes it an order of magnitude faster
 than `transpileModule`.
 
-To use it, first install `@swc/core` or `@swc/wasm`.  If using `importHelpers`, also install `@swc/helpers`.
+To use it, first install `@swc/core` or `@swc/wasm`.  If using `importHelpers`, also install `@swc/helpers`.  If `target` is less than "es2015" and using either `async`/`await` or generator functions, also install `regenerator-runtime`.
 
 ```shell
-npm i -D @swc/core @swc/helpers
+npm i -D @swc/core @swc/helpers regenerator-runtime
 ```
 
 Then add the following to your `tsconfig.json`.