docs: more explanation

[thiswillbeyourgithub]

Hi, I'm interested in mwmbl in the long term and considered helping to crawl at my amateur scale but the fact that I have no idea how the crawler works is making me uneasy. How secure is it ? What's the expected memory or bandwidth footprint? How does it work? Etc :)

[thiswillbeyourgithub]

Almost a year later and I'm still interested in contributing by crawling but not before this documentation has been adressed :)