Add a README

[eqrion]

It would be nice to have a README that explains:

1. The live link for the latest version (I think on netlify at least for now)
2. Running the benchmark in the browser
3. Running the benchmark in various shells
4. Available flags/query parameters
   - I always have to look at the source to remember the query params, would be nice to have a reference somewhere.

[danleh]

Agreed, and we should also update in-depth.html, e.g., it's missing https://webkit-jetstream-preview.netlify.app/in-depth.html#Dart-flute-todomvc-wasm and many of the other new workloads (not time critical though, just before the release).

[danleh]

Ah, for updating the workload descriptions, we already have issue #199.
For the README, #258 is a starting point.