README: add Changes section to document the low-level API changes

README.rst

@@ -37,3 +37,14 @@ Features
 * Secret-key encryption
 * Public-key encryption
 * HMAC (coming soon)
+
+
+Changes
+-------
+
+* 0.3.0: the low-level API (`nacl.c.*`) has been changed to match the
+  upstream NaCl C/C++ conventions (as well as those of other NaCl bindings).
+  The order of arguments and return values has changed significantly. If you
+  have code which calls these functions (e.g. `nacl.c.crypto_box_keypair()`),
+  you must review the new docstrings and update your code to match the new
+  conventions.