Moved a code snippet from cdef_classes.rst to the examples directory. Added...

Moved a code snippet from cdef_classes.rst to the examples directory. Added cdef readonly to show what it does.

docs/examples/tutorial/cdef_classes/wave_function.pyx

+from sin_of_square cimport Function
+
+cdef class WaveFunction(Function):
+
+    # Not available in Python-space:
+    cdef double offset
+
+    # Available in Python-space:
+    cdef public double freq
+
+    # Available in Python-space, but only for reading:
+    cdef readonly double scale
+
+    # Available in Python-space:
+    @property
+    def period(self):
+        return 1.0 / self.freq
+
+    @period.setter
+    def period(self, value):
+        self.freq = 1.0 / value

docs/src/tutorial/cdef_classes.rst

@@ -130,18 +130,4 @@ Attributes in cdef classes behave differently from attributes in regular classes
  - Attributes are by default only accessible from Cython (typed access)
  - Properties can be declared to expose dynamic attributes to Python-space
 
-::
-
-  cdef class WaveFunction(Function):
-      # Not available in Python-space:
-      cdef double offset
-      # Available in Python-space:
-      cdef public double freq
-      # Available in Python-space:
-      @property
-      def period(self):
-          return 1.0 / self.freq
-      @period.setter
-      def period(self, value):
-          self.freq = 1.0 / value
-      <...>
+.. literalinclude:: ../../examples/tutorial/cdef_classes/wave_function.pyx