Additional things to mention in the README

[baltpeter]

With #7, I've added a basic README. Here's a few things that I think we should also go into but didn't yet due to time constraints:

• [x] Explain the adapter format and how to contribute an adapter.
• [x] Mention tracker-wiki once that's ready.
• [ ] Explain the flow of how requests are parsed (decode -> data paths).

[baltpeter]

With #13, we're mostly there but:

I am especially wondering if you think this README still need another explanation of how the data extraction works in its own section (and not "Contributing adapters").

Well, I would expect that a significantly larger portion of people reading the README will be interested in a general idea of how TrackHAR works compared to people who actually want to contribute. So, I'd say yes. But that doesn't really have to happen now, we probably have more important things to do (and the information is pretty much all there already…). If we do add this at some point, we should probably split the contributing section into a separate CONTRIBUTING.md to not overwhelm people.