README & HACKING clean ups, and move gpl.txt to LICENSE

aa188ec shouldn't be controversial; just changing "rfcs" links to "doc".

bcdee99 is just the result of running the text in README and HACKING through Vim's `gq` motion with `tw=80`; i.e. breaking all the lines at 80 characters.

ea83234 makes many changes to the README. I merely fixed the grammar in some sentences. I fixed and standardized the Markdown formatting in other places. But in a few spots (particularly "Why", "Security", "Further Reading" and "Admin") I made quite extensive changes. In retrospect, maybe I should have separated this commit further. Anyway, if all those changes aren't to your liking, please let me know and I'll revert them. It would be a shame for this commit to go to waste.

9588e20 just moves the GPL license file to LICENSE, as is the standard with most open-source projects these days (on GitHub, at least).

See the changes rendered at: https://github.com/zenmunki/cjdns/blob/master/README.md

Cheers!