docs/unix: Add a new new quickref page for the UNIX port.

This adds a new quickstart page for the UNIX port that documents the
command line options and environment variables.

docs/index.rst

@@ -12,3 +12,4 @@ MicroPython documentation and references
     esp8266/quickref.rst
     esp32/quickref.rst
     wipy/quickref.rst
+    unix/quickref.rst

docs/unix/quickref.rst

@@ -0,0 +1,89 @@
+.. _unix_quickref:
+
+Quick reference for the UNIX and Windows ports
+==============================================
+
+Command line options
+--------------------
+
+Usage::
+
+    micropython [ -i ] [ -O<level> ] [ -v ] [ -X <option> ] [ -c <command> | -m <module> | <script> ] [ <args> ]
+
+
+Invocation options:
+
+.. option:: -c <command>
+
+   Runs the code in ``<command>``. The code can be one or more Python statements.
+
+.. option:: -m <module>
+
+   Runs the module ``<module>``. The module must be in ``sys.path``.
+
+.. option:: <script>
+
+   Runs the file ``<script>``. The script must be a valid MicroPython source
+   code file.
+
+If none of the 3 options above are given, then MicroPython is run in an
+interactive REPL mode.
+
+
+.. option:: <args>
+
+    Any additional arguments after the module or script will be passed to
+    ``sys.argv`` (not supported with the :option:`-c` option).
+
+
+General options:
+
+.. option:: -i
+
+    Enables inspection. When this flag is set, MicroPython will enter the
+    interactive REPL mode after the command, module or script has finished.
+    This can be useful for debugging the state after an unhandled exception.
+    Also see the :envvar:`MICROPYINSPECT` environment variable.
+
+.. option:: -O | -O<level> | -OO...
+
+    Sets the optimization level. The ``O`` can be followed by a number or can
+    be repeated multiple times to indicate the level. E.g. ``-O3`` is the same
+    as ``-OOO``.
+
+.. option:: -v
+
+    Increases the verbosity level. This option can be given multiple times.
+    This option only has an effect if ``MICROPY_DEBUG_PRINTERS`` was enabled
+    when MicroPython itself was compiled.
+
+.. option:: -X <option>
+
+    Specifies additional implementation-specific options. Possible options are:
+
+    - ``-X compile-only`` compiles the command, module or script but does not
+      run it.
+    - ``-X emit={bytecode,native,viper}`` sets the default code emitter. Native
+      emitters may not be available depending on the settings when MicroPython
+      itself was compiled.
+    - ``-X heapsize=<n>[w][K|M]`` sets the heap size for the garbage collector.
+      The suffix ``w`` means words instead of bytes. ``K`` means x1024 and ``M``
+      means x1024x1024.
+
+
+
+Environment variables
+---------------------
+
+.. envvar:: MICROPYPATH
+
+    Overrides the default search path for MicroPython libraries. ``MICROPYPATH``
+    should be set to a colon separated list of directories. If ``MICROPYPATH`` is
+    not defined, the search path will be ``~/.micropython/lib:/usr/lib/micropython``
+    or the value of the ``MICROPY_PY_SYS_PATH_DEFAULT`` option if it was set
+    when MicroPython itself was compiled.
+
+.. envvar:: MICROPYINSPECT
+
+    Enables inspection. If ``MICROPYINSPECT`` is set to a non-empty string, it
+    has the same effect as setting the :option:`-i` command line option.