Add readme example with conversion from array, async map

[karlhorky]

Hi there! First of all, thanks so much for this proposal, looks really cool!

I wanted to add an example with async mapping (which may be interesting to users of @sindresorhus' p-map and similar).

Credit to @Jamesernator : tc39/proposal-async-iteration#134 (comment)

[karlhorky]

Tests are failing because of an npm error about missing script: test:

0.25s$ npm test
npm ERR! missing script: test
npm ERR! A complete log of this run can be found in:
npm ERR!     /home/travis/.npm/_logs/2021-02-02T10_57_01_088Z-debug.log
The command "npm test" exited with 1.

https://travis-ci.org/github/tc39/proposal-iterator-helpers/builds/757218014

I guess the way to disable it is to specify script in .travis.yml, will add a commit for that.

[karlhorky]

Thanks for the merge @codehag ! 🙌