Document the fact that Node version >= 12 is needed.

[michielbdejong]

This line:
https://github.com/solid/solid-ui/blame/master/src/authn/authn.ts#L477
uses:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/flat
Which will not work in older browsers, nor in node 8 or node 10.
I updated the .travis.yml now to only run the tests in node 12. The .nvmrc version also says to use Node 12, but we should mention it in the readme.

Apart from the node version, we also need to decide which browsers we want to target, because if we say Array.flat is allowed, then we miss out on 15% of web users according to https://caniuse.com/#feat=array-flat ?

[michielbdejong]

Apart from Array.flat which requires Node >= 12, the tests use jsdom, which requires Node >= 10.

[megoth]

We do use preset-env that defaults to browserlist, which defaults to > 0.5%, last 2 versions, Firefox ESR, not dead. In other words, the code will be transpiled and polyfilled when necessary.

That said, I think we would benefit from being more specific about which browsers and environments that we want to support.