Adds importmap installation guide under install section

[thatosmk]

Changes

This PR adds a guide on installing stimulus-flatpickr with import maps.

Example

./bin/importmap pin flatpickr stimulus-flatpickr@beta

Pinning "flatpickr" to https://ga.jspm.io/npm:flatpickr@4.6.13/dist/esm/index.js
Pinning "stimulus-flatpickr" to https://ga.jspm.io/npm:stimulus-flatpickr@1.4.0/dist/index.js
Pinning "stimulus" to https://ga.jspm.io/npm:stimulus@3.2.2/dist/stimulus.js

You can then use it in your application.js entrypoint like you would any other module.

[adrienpoly]

thanks