examples: Print a description of the program to the console on startup.

examples/renderer/01-clear/README.txt

@@ -0,0 +1,3 @@
+This example code creates an SDL window and renderer, and then clears the
+window to a different color every frame, so you'll effectively get a window
+that's smoothly fading between colors.

examples/renderer/02-primitives/README.txt

@@ -0,0 +1,3 @@
+This example creates an SDL window and renderer, and then draws some lines,
+rectangles and points to it every frame.
+

examples/renderer/02-primitives/renderer-primitives.c

@@ -1,6 +1,6 @@
 /*
- * This example code creates an SDL window and renderer, and then draws a few
- * lines and rectangles to it every frame.
+ * This example creates an SDL window and renderer, and then draws some lines,
+ * rectangles and points to it every frame.
  *
  * This code is public domain. Feel free to use it for any purpose!
  */