message: Link to language reference from README
diff: @@ -5,7 +5,7 @@ Bril (the Big Red Intermediate Language) is a compiler IR made for teaching [CS
  It is an extremely simple instruction-based IR that is meant to be extended.
  Its canonical representation is JSON, which makes it easy to build tools from scratch to manipulate it.
  -This repository contains some infrastructure for Bril.
  +This repository contains the [language reference document][langref] and some infrastructure for Bril.
  These things are written in TypeScript:
  - A definition of the JSON format in `bril.ts`.
  @@ -14,6 +14,8 @@ These things are written in TypeScript:
  And there is also a parser and dumper for a human-readable and -writable text format, written in Python, under `bril-txt`.
  +[langref]: https://github.com/sampsyo/bril/blob/master/langref.md
  +
  Install the Tools
  -----------------
  