Add doc for tracer.trace

docs/grammar/tracer.md

@@ -0,0 +1,20 @@
+### tracer#trace()
+
+Whether you are using the CLI or the API, as long as you pass the `trace` option, the generated parser can trace it's progress by calling `tracer.trace( event )`, which can then be overwritten by passing an alternative tracer in the options to the generated parser.
+
+You can also optionally disable the default tracer by passing `features: { DefaultTracer: false }` to `peg.generate` or `peg.compiler.compile`, in which case a empry function will be called by default unless a tracer is passed to the parser.
+
+The `event` passed to the `trace` method contains the following properties:
+
+property | description
+--- | ---
+type | name of the event
+rule | rule that this event was emitted from
+result | the result of the match
+location | location in the grammar when the event was emitted
+
+The `type` property will only be 1 of 3 values:
+
+1. `rule.enter` when entering a rule
+2. `rule.match` when matching a rule was successful
+3. `rule.fail` when matching a rule failed