Update README.md

[qmonnet]

The README.md file is somewhat outdated. In particular:

• It should mention the systems rbpf can be compiled on (Linux/MacOS, Windows as well but without the JIT (see Make JIT usable on Windows #21)).
• It does not mention the API created some time ago to build the BPF programs.
• There are probably a lot of other items to update.

If you stumble on something else, do not hesitate to add elements to update below, so that I do not forget them the day I dive into this!

[jackcmay]

(let me know if I should ask this question someplace other than this issue)

What API to build BPF programs are you referring to?

[qmonnet]

No problem :)

That would be this one.

[qmonnet]

Well maybe there is not that much to change after all. I pushed a couple of commit in that direction:

• ba86c52
• 6691d23
• ba86c52

And I believe we're mostly good. I ran the tests and they still pass (I just pushed one fix). Another update will be needed when we merge the PR for returning errors from the verifier, but I pushed the change to the related branch already... So let's close here. Everyone, feel free to reopen if you spot other things to change or update on the README.