Fix supported error in extensions and add example

Author: lmariscal

README.md

@@ -65,7 +65,19 @@ The PRs and new feature development will occur in each binding's repo.
 ```nim
 import opengl
 
-# TODO
+# Create a context (GLFW recommended)
+
+if glInit():
+  echo "OpenGL loaded correctly."
+
+glClearColor(0.68f, 1f, 0.34f, 1f)
+glClear(GL_COLOR_BUFFER_BIT)
+
+# You can load extensions
+
+loadGL_ARB_gl_spirv()
+if glSpecializeShaderARB == nil:
+  echo "Extension not available"
 ```
 
 Check out the references and doc in order to understand OpenGL usage.