Explain label scheme used

[jbenet]

we should have a doc that explains the label scheme. so far it's basically:

• blue: topic / subsystem
• light red (important topic): security things
• red (very important): bugs!!
• orange (important): cleanup / refactor things
• light green: the difficulty level
• dark green: testing
• yellow: documentation
• gray waffle labels: used by https://waffle.io/ipfs/ipfs

notes:

• it's very ad-hoc, so no strict science.
• name should be understandable by themselves. for example, use "difficulty: medium" instead of "medium".
• use color to denote groupings, and perhaps some meaning (e.g. red is {bugs, problems, errors, races})

[jbenet]

https://robinpowered.com/blog/best-practice-system-for-organizing-and-tagging-github-issues/ <-- nice example doc. we probably can borrow from that-- but not wholesale as we have different reqs.