Clarify the status of stability

[vkrol]

Hello!

And now Node 13.2.0 has shipped. ES module support is no longer experimental. You conclude you've pushed it off long enough. The time has come.

This is not true.

Stability: 1 - Experimental

Expect major changes in the implementation including interoperability support, specifier resolution, and default behavior.

Experimental support for ECMAScript modules is enabled by default.

Source: https://nodejs.org/dist/latest-v13.x/docs/api/esm.html#esm_ecmascript_modules.