added documentation docs/README-os2.md (based on original readme.os2).

docs/README-os2.md

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+Simple DirectMedia Layer 2 for OS/2 & eComStation
+================================================================================
+SDL port for OS/2, authored by Andrey Vasilkin <digi@os2.snc.ru>, 2016
+
+
+OpenGL, joystick and audio capture not supported by this port.
+
+Additional environment variables (optional) for OS/2 version:
+
+SDL_AUDIO_SHARE
+  Values: 0 or 1, default is 0
+  Initializes the device as shareable or exclusively acquired.
+
+SDL_VIDEODRIVER
+  Values: DIVE or VMAN, default is DIVE
+  Use video subsystem: Direct interface video extensions (DIVE) or
+  Video Manager (VMAN).
+
+You may significantly increase video output speed with OS4 kernel and patched
+files vman.dll and dive.dll or with latest versions of ACPI support and video
+driver Panorama.
+
+Latest versions of OS/4 kernel:
+  http://gus.biysk.ru/os4/
+ (Info: https://www.os2world.com/wiki/index.php/Phoenix_OS/4)
+
+Patched files vman.dll and dive.dll:
+  http://gus.biysk.ru/os4/test/pached_dll/PATCHED_DLL.RAR
+
+
+Compiling:
+----------
+
+Open Watcom 1.9 or newer is tested. For the new Open Watcom V2 fork, see:
+https://github.com/open-watcom/ and https://open-watcom.github.io
+WATCOM ervironment variable must to be set to the Open Watcom install
+directory. To compile, run: wmake -f Makefile.os2
+
+
+Installing:
+-----------
+
+- eComStation:
+
+  If you have previously installed SDL2, make a Backup copy of SDL2.dll
+  located in D:\ecs\dll (where D: is disk on which installed eComStation).
+  Stop all programs running with SDL2. Copy SDL2.dll to D:\ecs\dll
+
+- OS/2:
+
+  Copy SDL2.dll to any directory on your LIBPATH.  If you have a previous
+  version installed, close all SDL2 applications before replacing the old
+  copy.  Also make sure that any other older versions of DLLs are removed
+  from your system.