Add CHANGELOG entries for 5.1 release

Probably a superset of what we actually want, but should be complete.

CHANGELOG.md

@@ -1,8 +1,46 @@
+### citus v5.1.0 (May 17, 2016) ###
+
+* Adds distributed COPY to rapidly populate distributed tables
+
+* Adds support for using EXPLAIN on distributed queries
+
+* Recognizes and fast-paths single-shard SELECT statements automatically
+
+* Increases INSERT throughput via shard pruning optimizations
+
+* Improves planner performance for joins involving tables with many shards
+
+* Adds ability to pass columns as arguments to function calls in UPDATEs
+
+* Introduces transaction manager for use by multi-shard commands
+
+* Adds COUNT(DISTINCT ...) pushdown optimization for hash-partitioned tables
+
+* Adds support for certain UNIQUE indexes on hash- or range-partitioned tables
+
+* Deprecates \stage in favor of using COPY for append-partition tables
+
+* Deprecates `copy_to_distributed_table` in favor of first-class COPY support
+
+* Fixes build problems when using non-packaged PostgreSQL installs
+
+* Fixes bug that sometimes skipped pruning when partitioned by a VARCHAR column
+
+* Fixes bug impeding use of user-defined functions in repartitioned subqueries
+
+* Fixes bug involving queries with equality comparisons of boolean types
+
+* Fixes crash that prevented use alongside `pg_stat_statements`
+
+* Fixes crash arising from SELECT queries that lack a target list
+
+* Improves warning and error messages
+
 ### citus v5.1.0-rc.2 (May 10, 2016) ###
 
-* Fix test failures
+* Fixes test failures
 
-* Fix EXPLAIN output when FORMAT JSON in use
+* Fixes EXPLAIN output when FORMAT JSON in use
 
 ### citus v5.1.0-rc.1 (May 4, 2016) ###