Feature: Add a license

[khawkins98]

Hi. I saw in package.json that you've indicated an ISC license. But it's always hard to know if that's intentional or perhaps a typo:

https://github.com/trys/eleventy-design-system/blob/ac36225f2e676e18639ac82bc2785e1aff9fa80a/package.json#L12

Assuming it is intentional, I'll suggest adding a license file so that, you know, people like me won't bother you in the future :)

In this PR I've used the ISC template + added your name and the year.

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[khawkins98]

Argh, I also based it off my branch with the code from PR 1. Hopefully both commits are agreable.