updated opencv_ffmpeg build instructions

3rdparty/ffmpeg/make.bat

-rem gcc -Wall -shared -o opencv_ffmpeg.dll -O3 -x c++ -I../include -I../include/ffmpeg_ -I../../modules/highgui/src ffopencv.c -L../lib -lavformat -lavcodec -lavdevice -lswscale -lavutil -lwsock32 -lpthreadGE2
-rem set path=c:\dev\msys32\bin;%path% & gcc -Wall -shared -o opencv_ffmpeg.dll -O2 -x c++ -I../include -I../include/ffmpeg_ -I../../modules/highgui/src ffopencv.c -L../lib -lavformat -lavcodec -lavdevice -lswscale -lavutil -lwsock32
+set path=c:\dev\msys32\bin;%path% & gcc -Wall -shared -o opencv_ffmpeg.dll -O2 -x c++ -I../include -I../include/ffmpeg_ -I../../modules/highgui/src ffopencv.c -L../lib -lavformat -lavcodec -lavdevice -lswscale -lavutil -lwsock32
 set path=c:\dev\msys64\bin;%path% & gcc -m64 -Wall -shared -o opencv_ffmpeg_64.dll -O2 -x c++ -I../include -I../include/ffmpeg_ -I../../modules/highgui/src ffopencv.c -L../lib -lavformat64 -lavcodec64 -lavdevice64 -lswscale64 -lavutil64 -lavcore64 -lwsock32 -lws2_32
\ No newline at end of file

3rdparty/ffmpeg/readme.txt

@@ -7,3 +7,38 @@ Otherwise the produced DLL will likely be dependent on libgcc_s_dw2-1.dll or sim
 While we want to make the DLLs with minimum dependencies: Win32 libraries + msvcrt.dll.
 
 ffopencv.c is really a C++ source, hence -x c++ is used.
+
+How to update opencv_ffmpeg.dll and opencv_ffmpeg_64.dll when a new version of FFMPEG is release?
+
+1. Install 32-bit MinGW + MSYS from
+   http://sourceforge.net/projects/mingw/files/Automated%20MinGW%20Installer/mingw-get-inst/
+   Let's assume, it's installed in C:\MSYS32.
+2. Install 64-bit MinGW. http://mingw-w64.sourceforge.net/
+   Let's assume, it's installed in C:\MSYS64
+3. Copy C:\MSYS32\msys to C:\MSYS64\msys. Edit C:\MSYS64\msys\etc\fstab, change C:\MSYS32 to C:\MSYS64.
+   
+4. Now you have working MSYS32 and MSYS64 environments.
+   Launch, one by one, C:\MSYS32\msys\msys.bat and C:\MSYS64\msys\msys.bat to create your home directories.
+
+4. Download ffmpeg-x.y.z.tar.gz (where x.y.z denotes the actual ffmpeg version).
+   Copy it to C:\MSYS{32|64}\msys\home\<loginname> directory.
+
+5. To build 32-bit ffmpeg libraries, run C:\MSYS32\msys\msys.bat and type the following commands:
+
+   5.1. tar -xzf ffmpeg-x.y.z.tar.gz
+   5.2. mkdir build
+   5.3. cd build
+   5.4. ../ffmpeg-x.y.z/configure --enable-w32threads
+   5.5. make
+   5.6. make install
+   5.7. cd /local/lib
+   5.8. strip -g *.a
+
+6. Then repeat the same for 64-bit case. The output libs: libavcodec.a etc. need to be renamed to libavcodec64.a etc.
+
+7. Then, copy all those libs to <opencv>\3rdparty\lib\, copy the headers to <opencv>\3rdparty\include\ffmpeg_.
+
+8. Then, go to <opencv>\3rdparty\ffmpeg, edit make.bat
+   (change paths to the actual paths to your msys32 and msys64 distributions) and then run make.bat
+
+