Add examples for flags in README

[futtetennista]

I couldn't find any documentation of flags and I ended up looking at the code. It wasn't clear to me from the current docs:

1. what was the top-level object name
2. wether individual flags had to be specified as objects or a list

• [x] Did you add a new feature to hpack? No
• [x] Did you add an acceptance test for that new feature to test/EndToEndSpec.hs? Not applicable

[ad-si]

@sol Can we please merge this? I was exactly looking for such an example snippet in the readme just now.