dev docs: Create HACKING.md.

Review-by: Matthew Martin

HACKING.md

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+Hacking on zsh-syntax-highlighting itself
+=========================================
+
+This document includes information for people working on z-sy-h itself: on the
+core driver (`zsh-syntax-highlighting.zsh`), on the highlighters in the
+distribution, and on the test suite.  It does not target third-party
+highlighter authors (although they may find it an interesting read).
+
+The 'main' highlighter
+----------------------
+
+The following function `pz` is useful when working on the `main` highlighting:
+
+    pq() {
+      (( $#argv )) || return 0
+      print -r -l -- ${(qqqq)argv}
+    }
+    pz() {
+      local arg
+      for arg; do
+        pq ${(z)arg}
+      done
+    }
+
+It prints, for each argument, its token breakdown, similar to how the main
+loop of the `main` highlighter sees it.
+
+IRC channel
+-----------
+
+We're on #zsh-syntax-highlighting on freenode.
+