Added simple build doc

BUILD.md

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+# vgmstream
+
+## Compilation requirements
+
+**GCC**: you need GCC and MAKE somewhere in path. In Windows this means one of these:
+- MinGW-w64 (32bit version): https://sourceforge.net/projects/mingw-w64/
+- MSYS2 with the MinGW-w64_shell (32bit) package: https://msys2.github.io/
+
+**MSVC / Visual Studio**: Visual Studio Community 2015 (free) should work:
+- Visual Studio: https://www.visualstudio.com/downloads/
+
+**Git**: optional, to generate version numbers:
+- Git for Windows: https://git-scm.com/download/win
+
+## Compiling modules
+
+### test.exe / in_vgmstream (Winamp)
+
+**With GCC**: use the *./Makefile* in the root folder, see inside for options. For compilation flags check *test/Makefile.mingw* or *winamp/Makefile.mingw*.
+You need to manually rebuild if you change a *.h* file (use *make clean*).
+
+In Linux you may need to use *Makefile.unix.am* instead, and note that some Linux makefiles aren't up to date.
+
+Windows CMD example for test.exe:
+```
+set PATH=%PATH%;C:\Git\usr\bin
+set PATH=%PATH%;C:\mingw-w64\i686-5.4.0-win32-sjlj-rt_v5-rev0\mingw32\bin
+
+cd vgmstream
+
+mingw32-make.exe mingw_test -f Makefile ^
+ VGM_ENABLE_FFMPEG=1 VGM_ENABLE_MAIATRAC3PLUS=0 ^
+ SHELL=sh.exe CC=gcc.exe AR=ar.exe STRIP=strip.exe DLLTOOL=dlltool.exe WINDRES=windres.exe
+```
+
+**With MSVC**: open *./vgmstream.sln* and compile in Visual Studio.
+
+### foo_input_vgmstream (foobar2000)
+Requires MSVC (foobar/SDK only links to MSVC C++ DLLs) and these dependencies:
+- foobar2000 SDK, in ../foobar/: http://www.foobar2000.org/SDK
+- WTL includes (if needed): http://wtl.sourceforge.net/
+- FDK-AAC, in ../fdk-aac/: https://github.com/kode54/fdk-aac
+- QAAC, in ../qaac/: https://github.com/kode54/qaac
+FDK-AAC/QAAC can be disabled by removing VGM_USE_MP4V2 and VGM_USE_FDKAAC.
+
+Open *./vgmstream.sln* as a base and add *fb2k/foo_input_vgmstream.vcxproj*, which expects the above.
+You may need to manually add includes and libs from WTL and ../foobar/foobar2000/shared/ in the compiler/linker options.
+VS2013 may not be compatible with the SDK.
+
+### xmp-vgmstream (XMPlay)
+Currently only MSVC is supported, though it should be compilable with GCC.
+
+Use *xmp-vgmstream/xmp-vgmstream.sln*; FDK-AAC/QAAC may be needed (see above).
+
+
+## Development
+
+### Structure
+vgmstream uses C (C89 when possible), except the foobar2000 plugin (C++).
+
+```
+./                   docs, scripts
+./ext_includes/      external includes for compiling
+./ext_libs/          external libs/DLLs for linking
+./fb2k/              foobar2000 plugin
+./src/               main vgmstream code and helpers
+./src/coding/        format sample decoders
+./src/layout/        format data demuxers
+./src/meta/          format header parsers
+./test/              test.exe CLI
+./unix/              Audacious plugin
+./winamp/            Winamp plugin
+./xmp-vgmstream/     XMPlay plugin
+```
+
+### Overview
+vgmstream works by parsing a music stream header (*meta/*), reading/demuxing data or looping (*layout/*) and decoding the compressed data into listenable PCM samples (*coding/*).
+
+Very simplified it goes like this:
+- player (test.exe, winamp plugin, etc) inits the stream *[main]*
+- init tries all parsers (metas) until one works *[init_vgmstream]*
+- parser reads header (channels, sample rate, loop points) and set ups a VGMSTREAM struct + layout/coding, if the format is correct *[init_vgmstream_(format-name)]*
+- player gets total_samples to play, based on the number of loops and other settings *[get_vgmstream_play_samples]*
+- player asks to fill a small sample buffer *[render_vgmstream]*
+- layout prepares bytes to read from the stream *[render_vgmstream_(layout)]*
+- decoder decodes bytes into PCM samples *[decode_vgmstream_(coding)]*
+- player plays those samples, asks to fill sample buffer, repeats until total_samples
+- layout moves back to loop_start when loop_end is reached *[vgmstream_do_loop]*
+
+### Adding new formats
+For new simple formats, assuming existing layout/coding:
+- *src/meta/(format-name).c*: create new format parser that reads all needed info from the stream header and inits VGMSTREAM
+- *src/meta/meta.h*: register parser's init
+- *src/vgmstream.h*: register new meta
+- *src/vgmstream.c*: add parser init to search list, add meta description
+- *winamp/in_vgmstream.c*
+  *fb2k/in_vgmstream.cpp*
+  *xml-vgmstream/DllMain.c*: add new extension to the format list
+- *src/Makefile*
+  *src/meta/Makefile.unix.am*
+  *src/libvgmstream.vcproj/vcxproj*: to compile new (format-name).c parser