update changelog

Author: myk002

changelog.txt

@@ -18,6 +18,10 @@ Template for new versions:
 
 # Future
 
+## Structures
+
+# 51.07-r1
+
 ## Structures
 - reorganize all structure definitions to match bay12 header layouts
 - promote all bay12 structures, enums, and bitfields to top-level types: this means that most ``T_*`` types now have top-level names