Note deprecations in README.

Author: mcordingley

README.md

@@ -112,6 +112,10 @@ If you need to combine together matrices, you can do so by calling the concatena
     - Drop deprecated functions and properties.
     - Tighten up interface with the `final` and `private` keywords.
 
+- 1.3.2
+    - Deprecate `__toString()` magic method.
+    - Deprecate `isSymmetric()`.
+
 - 1.3.1
     - Deprecate use of the `ArrayAccess` interface.
     - More internal code style fixes.