message: update rust documentation with source positions
diff: Rust Library
  ============
  -This is a no-frills interface between Bril's JSON and your [Rust][] code. It supports the [Bril core][core] along with the [SSA][], [memory][], [floating point][float], and [speculative execution][spec] extensions.
  +This is a no-frills interface between Bril's JSON and your [Rust][] code. It supports the [Bril core][core] along with the [SSA][], [memory][], [floating point][float], [speculative execution][spec], and [source positions][pos] extensions.
  Use
  ---
  @@ -12,7 +12,7 @@ Include this by adding the following to your `Cargo.toml`:
  [dependencies.bril-rs]
  version = "0.1.0"
  path = "../bril-rs"
  -features = ["ssa", "memory", "float", "speculate"]
  +features = ["ssa", "memory", "float", "speculate", "position"]
  ```
  Each of the extensions to [Bril core][core] is feature gated. To ignore an extension, remove its corresponding string from the `features` list.
  @@ -49,3 +49,4 @@ make features
  [memory]: ../lang/memory.md
  [float]: ../lang/float.md
  [spec]: ../lang/spec.md
  +[pos]: ../lang/syntax.md
  