README improvements

[TamaMcGlinn]

I think the README needs to list the installation instructions:

For linux (debian 9) I found these requirements:
sudo apt-get install esound

.... and then however to fix the below error (I am probably going to quickfix by removing joystick controls, for myself):

Linking C executable template
../SDL2/libSDL2.a(SDL_sysjoystick.c.o): In function `JoystickInitWithUdev':
/home/tama/projects/games/sdl2-template/SDL2/src/joystick/linux/SDL_sysjoystick.c:330: undefined reference to `SDL_UDEV_Init'
/home/tama/projects/games/sdl2-template/SDL2/src/joystick/linux/SDL_sysjoystick.c:335: undefined reference to `SDL_UDEV_AddCallback'
/home/tama/projects/games/sdl2-template/SDL2/src/joystick/linux/SDL_sysjoystick.c:341: undefined reference to `SDL_UDEV_Scan'
/home/tama/projects/games/sdl2-template/SDL2/src/joystick/linux/SDL_sysjoystick.c:336: undefined reference to `SDL_UDEV_Quit'
../SDL2/libSDL2.a(SDL_sysjoystick.c.o): In function `SDL_SYS_JoystickQuit':
/home/tama/projects/games/sdl2-template/SDL2/src/joystick/linux/SDL_sysjoystick.c:859: undefined reference to `SDL_UDEV_DelCallback'
../SDL2/libSDL2.a(SDL_sysjoystick.c.o): In function `SDL_SYS_JoystickDetect':
/home/tama/projects/games/sdl2-template/SDL2/src/joystick/linux/SDL_sysjoystick.c:381: undefined reference to `SDL_UDEV_Poll'
../SDL2/libSDL2.a(SDL_sysjoystick.c.o): In function `SDL_SYS_JoystickQuit':
/home/tama/projects/games/sdl2-template/SDL2/src/joystick/linux/SDL_sysjoystick.c:860: undefined reference to `SDL_UDEV_Quit'
collect2: error: ld returned 1 exit status
template/CMakeFiles/template.dir/build.make:87: recipe for target 'template/template' failed
make[2]: *** [template/template] Error 1
CMakeFiles/Makefile2:163: recipe for target 'template/CMakeFiles/template.dir/all' failed
make[1]: *** [template/CMakeFiles/template.dir/all] Error 2
Makefile:117: recipe for target 'all' failed
make: *** [all] Error 2```