Add ChangeLog entry

Add the ChangeLog entry describing the change.

ChangeLog

@@ -2,6 +2,10 @@ mbed TLS ChangeLog (Sorted per branch, date)
 
 = mbed TLS x.x.x branch released xxxx-xx-xx
 
+API Changes
+ * Add a common error code for a feature that is not supported by the
+ underlying alternative implementations.
+
 Features
  * Add support for temporarily suspending expensive ECC computations after
  some configurable amount of operations, to be used in single-threaded
@@ -24,6 +28,12 @@ Features
  hash and signature sizes that comply with FIPS 186-4, including SHA-512
  with a 1024-bit key.
 
+New deprecations
+ * All the current module specific errors that mean a feature is not available
+ are deprecated, so the platform error should be used.
+ * All the module specific generic hardware accelaration errors that existed
+ are deprecated, so the platform error should be used.
+
 Bugfix
  * Fix wrong order of freeing in programs/ssl/ssl_server2 example
  application leading to a memory leak in case both