Add C program and update README with usage information.

README.md

 # Find SDL2 shared library version
 
-This is a very simple program that lets you find out the version of a SDL2 shared library.
\ No newline at end of file
+This is a very simple program that lets you find the version of a SDL2 shared library.
+
+Compile: ```gcc `sdl2-config --libs --cflags` find_version.c -o find_version```
+
+Run against a specific shared library (example): `LD_PRELOAD=./libSDL2-2.0.so.0 ./find_version`
+
+Output:
+```
+Compiled against SDL version:	2.0.20
+Linking against SDL version:	2.0.14
+```
+The first line is the SDL2 version this program was compiled against. This is just for comparison.
+The second line is the SDL2 version of the shared library you specified in LD_PRELOAD.

find_version.c

+#include <stdio.h>
+#include <SDL.h>
+
+int main(int argc, char *argv[])
+{
+	SDL_version compiled;
+	SDL_version linked;
+
+	SDL_VERSION(&compiled);
+	SDL_GetVersion(&linked);
+
+	printf("Compiled against SDL version:\t%d.%d.%d\n", compiled.major, compiled.minor, compiled.patch);
+	printf("Linking against SDL version:\t%d.%d.%d\n", linked.major, linked.minor, linked.patch);
+
+	return 0;
+}