Document how to run tests locally

[walles]

The README.md should document how to run the test suite locally.

I've been failing at that for a while now, which makes contributing difficult.

Related to #3 perhaps?

I tried npm install && ./do test. The first part goes fine but ./do test just hangs here:

johan@Johans-hemdator rewrap-revived % ./do test
Building with Fable...

[walles]

What I tried locally seems to be what CI is doing as well, so I'm guessing the CI runs time out for the same reason my local ones do.

Rewrap/.github/workflows/main.yml

Line 26 in fd5fa4b

run: ./do build test --production