chore: add rds submodule (#893)

Signed-off-by: feng-tao <fengtao04@gmail.com>

.gitmodules

@@ -22,3 +22,6 @@
 	path = amundsengremlin
 	url = https://github.com/amundsen-io/amundsengremlin
 	branch = master
+[submodule "amundsenrds"]
+	path = amundsenrds
+	url = https://github.com/amundsen-io/amundsenrds.git

README.md

@@ -48,6 +48,7 @@ Amundsen is hosted by the [LF AI & Data Foundation](https://lfaidata.foundation/
   or with an [Airflow DAG](https://github.com/amundsen-io/amundsendatabuilder/tree/master/example/dags) importing the library. <img src="https://img.shields.io/github/commit-activity/w/amundsen-io/amundsendatabuilder.svg" />
 - [amundsencommon](https://github.com/amundsen-io/amundsencommon): Amundsen Common library holds common codes among microservices in Amundsen. <img src="https://img.shields.io/github/commit-activity/w/amundsen-io/amundsencommon.svg" />
 - [amundsengremlin](https://github.com/amundsen-io/amundsengremlin): Amundsen Gremlin library holds code used for converting model objects into vertices and edges in gremlin. It's used for loading data into an AWS Neptune backend. <img src="https://img.shields.io/github/commit-activity/w/amundsen-io/amundsengremlin.svg" />
+- [amundsenrds](https://github.com/amundsen-io/amundsenrds): Amundsenrds contains ORM models to support relational database as metadata backend store in Amundsen. The schema in ORM models follows the logic of databuilder models. Amundsenrds will be used in databuilder and metadatalibrary for metadata storage and retrieval with relational databases. <img src="https://img.shields.io/github/commit-activity/w/amundsen-io/amundsenrds.svg" />
 
 ## Homepage
 

amundsenrds @ d21da320

+Subproject commit d21da3207605e8a0cccc1da874c7b1915efb12d9