Add minimal documentation for const memory views.

CHANGES.rst

@@ -8,10 +8,6 @@ Cython Changelog
 Features added
 --------------
 
-* When compiling with gcc, the module init function is now tuned for small
-  code size instead of whatever compile flags were provided externally.
-  (Github issue #2102)
-
 * Cdef classes can now multiply inherit from ordinary Python classes.
   (The primary base must still be a c class, possibly ``object``, and
   the other bases must *not* be cdef classes.)
@@ -19,6 +15,15 @@ Features added
 * Type inference is now supported for Pythran compiled NumPy expressions.
   Patch by Nils Braun.  (Github issue #1954)
 
+* Read-only buffers are automatically supported for memoryviews if it can be
+  determined at compile time that the code does not need write access.
+  They can also be allowed explicitly by adding the ``const`` modifier to their
+  declaration.  (Github issues #1605, #1869)
+
+* When compiling with gcc, the module init function is now tuned for small
+  code size instead of whatever compile flags were provided externally.
+  (Github issue #2102)
+
 * C file includes are moved behind the module declarations if possible, to allow
   them to depend on module declarations themselves.
   Patch by Jeroen Demeyer.  (Github issue #1896)

docs/src/userguide/memoryviews.rst

@@ -227,6 +227,33 @@ As for NumPy, new axes can be introduced by indexing an array with ``None`` ::
 One may mix new axis indexing with all other forms of indexing and slicing.
 See also an example_.
 
+Const buffers
+-------------
+
+Since Cython 0.28, memoryviews can be declared as ``const`` to allow
+read-only buffers as input::
+
+    cdef const double[:] myslice
+
+    a = np.linspace(0, 10, num=50)
+    a.setflags(write=False)
+    myslice = a
+
+Note that this does not *require* the input to be read-only::
+
+    a = np.linspace(0, 10, num=50)
+    myslice = a
+
+Writable buffers will still be accepted by ``const`` views, but read-only
+buffers are not accepted for non-const, writable views.
+
+Cython will also request a read-only view automatically if it can determine
+at compile time that a writable buffer is not required.  The support for
+automatically distinguishing between buffer usage types, and the compile
+time correctness checks for read-only views are expected to further improve
+over time.
+
+
 Comparison to the old buffer support
 ====================================