Changelog entry for release v15.12

The faulty release v15.11 was withdrawn.

CHANGELOG.md

@@ -1,12 +1,17 @@
 # Release History
 
-## [v15.11](https://github.com/smuehlst/circle-stdlib/releases/tag/v15.11)
+## [v15.12](https://github.com/smuehlst/circle-stdlib/releases/tag/v15.12)
 
 * Synced with [Circle Step 44.4.1](https://github.com/rsta2/circle/releases/tag/Step44.4.1).
 * Updated mbed TLS to [release v2.16.12](https://github.com/ARMmbed/mbedtls/releases/tag/v2.16.12).
 * New `configure` command line option `--option` for specifying additional preprocessor defines
   consistently across all libraries.
 
+## v15.11
+
+Faulty release that was intended to include Circle step 44.4.1, but the
+commit was forgotten. Therefore release v15.11 is superseded by v15.12.
+
 ## [v15.10](https://github.com/smuehlst/circle-stdlib/releases/tag/v15.10)
 
 * Synced with [Circle Step 44.3](https://github.com/rsta2/circle/releases/tag/Step44.3).