Update mach from gecko tree

python/mach/docs/index.rst

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+====
+mach
+====
+
+Mach (German for *do*) is a generic command dispatcher for the command
+line.
+
+To use mach, you install the mach core (a Python package), create an
+executable *driver* script (named whatever you want), and write mach
+commands. When the *driver* is executed, mach dispatches to the
+requested command handler automatically.
+
+Features
+========
+
+On a high level, mach is similar to using argparse with subparsers (for
+command handling). When you dig deeper, mach offers a number of
+additional features:
+
+Distributed command definitions
+  With optparse/argparse, you have to define your commands on a central
+  parser instance. With mach, you annotate your command methods with
+  decorators and mach finds and dispatches to them automatically.
+
+Command categories
+  Mach commands can be grouped into categories when displayed in help.
+  This is currently not possible with argparse.
+
+Logging management
+  Mach provides a facility for logging (both classical text and
+  structured) that is available to any command handler.
+
+Settings files
+  Mach provides a facility for reading settings from an ini-like file
+  format.
+
+Components
+==========
+
+Mach is conceptually composed of the following components:
+
+core
+  The mach core is the core code powering mach. This is a Python package
+  that contains all the business logic that makes mach work. The mach
+  core is common to all mach deployments.
+
+commands
+  These are what mach dispatches to. Commands are simply Python methods
+  registered as command names. The set of commands is unique to the
+  environment mach is deployed in.
+
+driver
+  The *driver* is the entry-point to mach. It is simply an executable
+  script that loads the mach core, tells it where commands can be found,
+  then asks the mach core to handle the current request. The driver is
+  unique to the deployed environment. But, it's usually based on an
+  example from this source tree.
+
+Project State
+=============
+
+mach was originally written as a command dispatching framework to aid
+Firefox development. While the code is mostly generic, there are still
+some pieces that closely tie it to Mozilla/Firefox. The goal is for
+these to eventually be removed and replaced with generic features so
+mach is suitable for anybody to use. Until then, mach may not be the
+best fit for you.
+
+.. toctree::
+   :maxdepth: 1
+
+   commands
+   driver
+   logging