docs: es6+, finally

[tbjgolden]

Added docs to clarify two things that nearly made me install request-promise instead of request-promise when I didn't necessarily want to

1. use ES6+ instead of ES6 as .finally(...) was added in ES9
2. pointed out that finally(...) is indeed included in Node v10+ native promises

[coveralls]

Coverage remained the same at 100.0% when pulling 239ce46 on tbjgolden:docs/clarify-es6-finally into 2ae195a on request:master.

[rafaeleyng]

Please accept this, is a simple documentation fix that will avoid a lot of confusion

[analog-nico]

Thanks a lot @tbjgolden ! And thanks for bumping this up @rafaeleyng !