update changelog

Signed-off-by: Nyudong.cai <yudong.cai@zilliz.com>

CHANGELOG.md

@@ -2,6 +2,9 @@
 
 Please mark all change in change log and use the issue from GitHub
 
+# Milvus 0.8.0 (TBD)
+
+
 # Milvus 0.7.0 (2020-03-11)
 
 ## Bug