update the spec change log with changes from v3.0.12 (#878)

* start to update the spec change log from v3.0.12

* added the cl_khr_external_memory issue to the change log

api/appendix_e.asciidoc

@@ -551,3 +551,13 @@ Changes from *v3.0.11*:
   * Specified error behavior when a command buffer is finalized multiple times.
   * Added new extension:
       ** `cl_khr_command_buffer_mutable_dispatch` (provisional)
+
+Changes from *v3.0.12*:
+
+  * Fixed the accuracy requirements description for half-precision math functions (those prefixed by `half_`).
+  * Clarified that the semaphore type must always be provided when creating a semaphore.
+  * Removed an unnecessary and contradictory error condition when creating a semaphore.
+  * Added an issue regarding non-linear image import to the `cl_khr_external_memory` extension.
+  * Added missing calls to {clBuildProgram} to the `cl_khr_command_buffer` and `cl_khr_command_buffer_mutable_dispatch` sample code.
+  * Fixed a copy-paste error in the extensions quick reference appendix.
+  * Fixed typos and improved formatting consistency in the extensions spec.