Add DLL documentation for install bundling

bin/Resources/windows-dll-list.txt

+# SRB2 - Which DLLs do I need to bundle?
+
+Updated 12/4/2018 (v2.1.21)
+
+Most required DLL binaries are in the libs\ folder. The binaries that are NOT there are living in bin\Resources.
+
+Here are the required DLLs, per build. For each architecture, copy all the binaries from these folders:
+
+* bin\Resources\[i686/x86_64]
+* libs\SDL2\[i686/x86_64]...\bin
+* libs\SDL2_mixer\[i686/x86_64]...\bin
+
+and don't forget to build r_opengl.dll for srb2dd.
+
+## srb2win, 32-bit
+
+* bin\Resources\i686\exchndl.dll
+* bin\Resources\i686\libgme.dll
+* bin\Resources\i686\mgwhelp.dll (depend for exchndl.dll)
+* libs\SDL2\i686-w64-mingw32\bin\SDL2.dll
+* libs\SDL2_mixer\i686-w64-mingw32\bin\*.dll (get everything)
+
+## srb2win, 64-bit
+
+* bin\Resources\x86_64\exchndl.dll
+* bin\Resources\x86_64\libgme.dll
+* bin\Resources\x86_64\mgwhelp.dll (depend for exchndl.dll)
+* libs\SDL2\x86_64-w64-mingw32\bin\SDL2.dll
+* libs\SDL2_mixer\x86_64-w64-mingw32\bin\*.dll (get everything)
+
+## srb2dd, 32-bit
+
+* bin\Resources\i686\exchndl.dll
+* bin\Resources\i686\fmodex.dll
+* bin\Resources\i686\libgme.dll
+* bin\Resources\i686\mgwhelp.dll (depend for exchndl.dll)
+* r_opengl.dll (build this from make)
+
+## srb2dd, 64-bit
+
+* bin\Resources\x86_64\exchndl.dll
+* bin\Resources\x86_64\fmodex.dll
+* bin\Resources\x86_64\libgme.dll
+* bin\Resources\x86_64\mgwhelp.dll (depend for exchndl.dll)
+* r_opengl.dll (build this from make)