mb: Allow overriding the path to the gyp script

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
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 You can pass the `-q/--quiet` flag to get mb to be silent unless there is an
 error, and pass the `-v/--verbose` flag to get mb to log all of the files
 that are read and written, and all the commands that are run.
 
 If the build config will use the Goma distributed-build system, you can pass
 the path to your Goma client in the `-g/--goma-dir` flag, and it will be
 incorporated into the appropriate flags for GYP or GN as needed.
 
 If gen ends up using GYP, the path must have a valid GYP configuration as the
 last component of the path (i.e., specify `//out/Release_x64`, not `//out`).
+The gyp script defaults to `//build/gyp_chromium`, but can be overridden with
+the `--gyp-script` flag, e.g. `--gyp-script=gypfiles/gyp_v8`.
 
 ### `mb help`
 
 Produces help output on the other subcommands
 
 ### `mb lookup`
 
 Prints what command will be run by `mb gen` (like `mb gen -n` but does
 not require you to specify a path).
 
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 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.