updated CHANGELOG

CHANGELOG

@@ -1,6 +1,38 @@
 v2.8.0 (XXXX-XX-XX)
 -------------------
 
+* added automatic deadlock detection for transactions
+
+  In case a deadlock is detected, a multi-collection operation may be rolled back
+  automatically and fail with error 29 (`deadlock detected`). Client code for
+  operations containing more than one collection should be aware of this potential
+  error and handle it accordingly, either by aborting the operation or retrying it.
+
+* fixed problem during SSL client connection abort that led to scheduler thread 
+  staying at 100% CPU saturation
+
+* Added C++ implementations for the AQL arithmetic operations and the following 
+  AQL functions:
+  - ABS
+  - APPEND
+  - COLLECTIONS
+  - CONTAINS
+  - DOCUMENT
+  - EDGES
+  - FIRST_DOCUMENT
+  - FIRST_LIST
+  - FLATTEN
+  - FLOOR
+  - MINUS
+  - NTH
+  - PARSE_IDENTIFIER
+  - POP
+  - PUSH
+  - RAND
+  - ROUND
+  - SQRT
+  - ZIP 
+
 * improved performance of skipping over many documents in an AQL query when no
   indexes and no filters are used, e.g.