Merge pull request #3481 from caternuson/iss3480_docupdate

Add MP const info to Design Guide

Author: tannewt

docs/design_guide.rst

@@ -485,6 +485,19 @@ struct.pack
 
 Use `struct.pack_into` instead of `struct.pack`.
 
+Use of MicroPython ``const()``
+--------------------------------------------------------------------------------
+The MicroPython ``const()`` feature, as discussed in `this forum post
+<https://forum.micropython.org/viewtopic.php?t=450>`_, and in `this issue thread
+<https://github.com/micropython/micropython/issues/573>`_, provides some
+optimizations that can be useful on smaller, memory constrained devices. However,
+when using ``const()``, keep in mind these general guide lines:
+
+- Always use via an import, ex: ``from micropython import const``
+- Limit use to global (module level) variables only.
+- If user will not need access to variable, prefix name with a leading
+  underscore, ex: ``_SOME_CONST``.
+
 Sensor properties and units
 --------------------------------------------------------------------------------