feat: updated CHANGELOG for 2020-07 release (#4452)

Author: devbww

CHANGELOG.md

@@ -1,6 +1,40 @@
 # Changelog
 
-## v1.15.0 - TBD
+## v1.16.0 - TBD
+
+## v1.15.0 - 2020-07
+
+### Bigtable
+
+* feature: add bigtable backup API support (#4407)
+* fix: deadlock in MutationBatcher (#4327)
+
+### Storage
+
+* fix: support ObjectReadStream::tellg() (#4402)
+* fix(storage): treat 408 errors as retryable (#4397)
+* fix(GCS+gRPC): simplify DirectPath configuration (#4388)
+* feat(GCS+gRPC): DirectPath can be manually configured (#4379)
+* fix: warnings with MSVC 2019 16.6 (aka 19.26 akak 14.26) (#4365)
+* feat(storage/benchmarks): compare to raw downloads (#4362)
+* feat(storage/benchmark): experiment integration test (#4360)
+* feat(storage/benchmarks): control CRC32C/MD5 options (#4326)
+
+### Spanner
+
+* fix: incorrect metadata key string (#4431)
+* doc(spanner): add CreateInstance() example (#4405)
+* fix(spanner): examples on how to delete data (#4401)
+* feat: use `SELECT 1` to refresh sessions (#4377)
+* fix(spanner): use correct name for test (#4373)
+
+### Common libraries
+
+* fix: CompletionQueue::RunAsync is always async (#4448)
+* fix: broken builds on Windows+CMake+Release (#4442)
+* fix: test with correct MockCompletionQueue (#4427)
+* fix: make potential narrowing cast of nanoseconds explicit (#4391)
+* fix: memory stomping in CompletionQueue::RunAsync (#4330)
 
 ## v1.14.0 - 2020-06