[skip ci]: add Snapshot design doc link (#2320)

Signed-off-by: Npeng.xu <peng.xu@zilliz.com>

DESIGN.md

@@ -8,3 +8,4 @@ The following list contains existing design documents for Milvus.
 -   [Support delete/search by ID, attribute filtering, ID de-duplication](https://docs.google.com/document/d/1CDKdTj_DnE90YaZrPgsMaphqOTkMdbKETNrsFKj_Bco/edit?usp=sharing)
 -   [Support write-ahead logging](https://docs.google.com/document/d/12N8RC_wJb2dvEKY9jrlh8hU_eH8jxQVBewoPuHNqcXE/edit?usp=sharing)
 -   [Support in-service config modification](https://docs.google.com/document/d/1pK1joWJgAHM5nVp3q005iLbLqU5bn9InWeBy0mRAoSg/edit?usp=sharing)
+-   [Support elementary MVCC](https://docs.google.com/document/d/1eUTqp_gZ4dlkyNv0FcCYg19ox5ZzdRshEf8SqYXP1Yw/edit?usp=sharing)