Add docs for @babel/register/experimental-worker (#2616)

Co-authored-by: Huáng Jùnliàng <jlhwung@gmail.com>

docs/register.md

@@ -143,4 +143,15 @@ Because it is your own code that triggered the load, and not the logic within
 `@babel/register` itself, this should successfully compile any plugin/preset
 that loads synchronously.
 
+## Experimental Babel 8 implementation
+
+You can also test the new experimental implementation that will be enabled by default in Babel 8, using
+```js
+require("@babel/register/experimental-worker");
+```
+
+It internally runs Babel asynchronously, so it's compatible with [`.mjs` configuration files](config-files.md#configuration-file-types). You can already use it as a replacement of `@babel/register` with a few caveats:
+- If you programmatically specify `@babel/register` options (using `require("@babel/register")({ /* ... options */ })`), you must make sure that they are serializable. This means that you cannot pass plugin functions defined inline, but you must move them to a separate `./my-plugin.js` file or to a `babel.config.js` file.
+- The new implementation is still experimental: it _should_ have the same features as the existing one, but there may be some new bugs and regressions.
+
 **Note:** `@babel/register` does _not_ support compiling native Node.js ES modules on the fly, since currently there is no stable API for intercepting ES modules loading.