update the HACKING.md to reflect better and/or git-based instructions, also a test commit for git migration.

by moving documents into a own docs directory.

this is specially important for our GitHub impression which I suspect most 
coders will have their first meeting with I2P on. The documentation is far from
done but it should make value for any new developer by this time so I've decided 
to push them. It losly describes our build systems, and has a monotone cheat sheet 
which always has an "git equalent" so it's easier to handle for people used to git.