doc: Add modding documentation

Omnispeak is now being used for some basic mods. Provide some simple
documentation about how best to make or port mods to Omnispeak.

This information is subject to change in future Omnispeak versions, but
should be a good start.

Operation Station Infiltration v1.0.2

See:
https://pckf.com/viewtopic.php?p=106764#p106764

Changelog:
Windows exe files are no longer compressed, to avoid some Antivirus false-positives.

New build system for Linux versions which works better on distros with older glibc versions.

(SDL2 is not included in the Linux version any more, as most systems should already have it.)

Changed the default sound sample rate to 44,100Hz (same as CD audio). This should be more compatible
than 49,716Hz, which was the old OPL2 rate. This can be reverted by adding 'sampleRate = 49716" to
OMNISPK.CFG. If you're still having issues, 48000 is another good rate to try.

New setting in OMNISPK.CFG "sd_sdl_audioDriver" to override the default sound driver. On windows, this
now defaults to "directsound", not "wasapi" (Vista+ new API). If neither of those work, "winmm" might,
though in my testing seemed a little worse.

Added support for stereo output (the game is still mono, but outputs two identical left/right channels),
which might be more compatible with some drivers. This is enabled by default, add "audioChannels = 1"
to OMNISPK.CFG to revert.

Version 1.0 of Operation Station Infiltration

This corresponds to the version of Omnispeak used in the Operation
Station Infiltration mod, version 1.0:
https://pckf.com/viewtopic.php?t=15448

Omnispeak v1.1

Celebrating the 30th anniversary of Commander Keen in Goodbye Galaxy,
this version of Omnispeak includes all of the new features and bugfixes
from last several years, including:
- QuickLoad/QuickSave features
- Improved Joystick / Gamepad support
- Support for real Adlib / OPL2 soundcards
- Support for the Nuked OPL3 emulator for more accurate sound emulation.
- Improved compatibility with windows systems (due to a new SDL2-based
  renderer)
- A new configuration file -- OMNISPK.CFG -- which saves the new
  options, and allows several tweaks.
- Support for running the game from different directories (and splitting
  the savegames/config data into a different directory) with the
  /GAMEPATH and /USERPATH options.
- Support for integer scaling for graphics.
- An experimental, buggy DOS port
- Several accuracy fixes and other bugfixes.
- And more!

Don't forget to check out the updated Omnispeak website at:
	https://davidgow.net/keen/omnispeak.html

Omnispeak v1.0 release