add apply_SDL_patch.md doc

docs/apply_SDL_patch.md

+# Apply SDL patch
+
+There are some bugs we have to change sdl library or we found it is a better to fix it inside sdl instead of fixing it in anbox.
+This patches are here:  _anbox/external/libSDL/_ . You can download sdl library(refer to section "Download SDL") to apply these patches(refer to section "Apply SDL patch") if needed.
+
+## Download SDL
+
+Currently, anbox based on SDL2, so our patches are based on SDL2. You can download the souce package here: 
+ _https://www.libsdl.org/release/SDL2-2.0.9.tar.gz_ 
+
+## Apply SDL patch
+
+```
+1. Unzip the files you downloaded
+2. cd <your dir>/libsdl2-2.0.9+dfsg1
+3. patch -p1 < <your dir>/<patch name>.patch
+4. dpkg-buildpackage -rfakeroot -b
+5. cd ..
+6. dpkg -i ./*.deb
+```
\ No newline at end of file