Fix "include" examples to use Github-flavored code block

Previously, these lines were using '>' which is the Markdown syntax
for a "blockquote", but within this, the '#' in "#include" was still
interpreted as a header, (which was not desired), and throwing away
the rest of the line.

It seems that a "codeblock" is what is wanted here instead, (which
should result in un-interpreted text wrapped in <pre>). Markdown
expects 4-space indent for this.

See:

	https://help.github.com/articles/github-flavored-markdown

README.md

@@ -30,20 +30,22 @@ as well as we think it could be done.
 
 Additionally, the proliferation of OpenGL ABIs (desktop GL, GLESv1,
 GLESv2) and window systems (GLX, AGL, WGL, all versus EGL) means that
+an individual developer needs to know more and more about how to load
+their classes symbols.
 
 Switching your code to using epoxy
 ----------------------------------
 
 It should be as easy as replacing:
 
-> #include <GL/gl.h>
+    #include <GL/gl.h>
-> #include <GL/glx.h>
+    #include <GL/glx.h>
-> #include <GL/glext.h>
+    #include <GL/glext.h>
 
 with:
 
-> #include <epoxy/gl.h>
+    #include <epoxy/gl.h>
-> #include <epoxy/glx.h>
+    #include <epoxy/glx.h>
 
 Additionally, some new helpers become available, so you don't have to
 write them: