[urbit] Hoon Reference Card

[rsaarelm]

I've made a plaintext quick reference card for Hoon runes:
https://github.com/rsaarelm/hoon-refcard/blob/c057b8f0286304e60a5cf43dc58a940610f6bafb/hoon-ref.txt

I'm still learning Hoon, and made this based on the current rune docs, so
there are likely to be errors and bits that could be put better in it. The
project directory has a Makefile for generating a PDF from it using
enscript.

[c***@gmail.com]

Sweet! There are some Book constructs that don't map to runes, though. Like explaining +< tree navigation, or how `a becomes [~ a]. That might be out of scope for it, though.