Add new build targets. Update README and gitignore.

Added `make clean`
Added `make hello` to show Hello SDL

Ported from ninenines#1
Part of the change was left out because there is no longer a secondary
Makefile in c_src

.gitignore

@@ -4,3 +4,4 @@ ebin
 esdl2.d
 examples/*/*.beam
 priv
+.DS_Store

Makefile

@@ -33,3 +33,14 @@ endif
 bullet_engine:: all
 	erlc -o examples/bullet_engine examples/bullet_engine/*.erl
 	cd examples/bullet_engine && ./start.sh
+
+hello::
+	erlc -o examples/hello_sdl examples/hello_sdl/*.erl
+	cd examples/hello_sdl && ./start.sh
+
+clean::
+	@rm -f *.swp
+	@rm -f c_src/env.mk
+	@rm -rf ebin/
+	@rm -f examples/*/*.beam
+	@echo clean

README.md

@@ -1,19 +1,29 @@
 ESDL2
 =====
 
-SDL2 Erlang NIF.
+SDL2 Erlang NIFs
+
+SDL provides cross-platform low level access to graphics, audio, keyboard, mouseand joystick hardware via OpenGL and Direct3D. This is an Erlang binding. http://www.libsdl.org/
 
 Status
 ------
 
 Week-end project. Work in progress.
-
-The following limitations apply:
-
+
+ Building
+ --------
+
  *  Erlang 17.0+ is required
- *  SDL 2.0.3+ is required
+ *  SDL 2.0.3+ is required (`http://www.libsdl.org/download-2.0.php`)
+ *  SDL\_image is required (`https://www.libsdl.org/projects/SDL_image/`)
  *  No support for UTF-8 strings, only Latin-1
 
+Tip: configure SDL\_image with --disable-webp for not having to install libwebp.
+
+Build using `make`.
+
+Ideas
+-----
 The following ideas need to be investigated:
 
  *  We may benefit from the reference receive optimization when doing calls