Add an `mb audit` command to track the GYP->GN progress.

tools/mb/docs/user_guide.md

 # The MB (Meta-Build wrapper) user guide
 
 [TOC]
 
 ## Introduction
 
 `mb` is a simple python wrapper around the GYP and GN meta-build tools to
 be used as part of the GYP->GN migration.
 
 It is intended to be used by bots to make it easier to manage the configuration
@@ skipping 75 matching lines @@
        need to be pruned).
 
 See [design_spec.md](the design spec) for more details and examples; the
 differences can be subtle.  We won't even go into how the `targets` and
 `build_targets` differ from each other or from `compile_targets` and
 `test_targets`.
 
 The `-b/--builder`, `-c/--config`, `-f/--config-file`, `-m/--master`,
 `-q/--quiet`, and `-v/--verbose` flags work as documented for `mb gen`.
 
+### `mb audit`
+
+`mb audit` is used to track the progress of the GYP->GN migration. You can
+use it to check a single master, or all the masters we care about. See
+`mb help audit` for more details (most people are not expected to care about
+this).
+
 ### `mb gen`
 
 `mb gen` is responsible for generating the Ninja files by invoking either GYP
 or GN as appropriate. It takes arguments to specify a build config and
 a directory, then runs GYP or GN as appropriate:
 
 ```
 % mb gen -m tryserver.chromium.linux -b linux_rel //out/Release
 % mb gen -c linux_rel_trybot //out/Release
 ```
@@ skipping 166 matching lines @@
 run than you expect; running `'mb -v'` will print what it's doing and
 run the commands; `'mb -n'` will print what it will do but *not* run
 the commands.
 
 If you hit weirder things than that, add some print statements to the
 python script, send a question to gn-dev@chromium.org, or
 [file a bug](https://crbug.com/new) with the label
 'mb' and cc: dpranke@chromium.org.