feat: add amundsen gremlin (#743)

Signed-off-by: Joshua Hoskins <hoskins@squareup.com>

.gitmodules

@@ -17,3 +17,6 @@
 [submodule "amundsencommon"]
 	path = amundsencommon
 	url = https://github.com/amundsen-io/amundsencommon
+[submodule "amundsengremlin"]
+	path = amundsengremlin
+	url = git@github.com:amundsen-io/amundsengremlin.git

README.md

@@ -41,6 +41,7 @@ Amundsen is hosted by the [LF AI Foundation](https://lfdl.io)(LF AI). It include
   Users could either load the data with [a python script](https://github.com/amundsen-io/amundsendatabuilder/blob/master/example/scripts/sample_data_loader.py) with the library
   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" />
 
 ## Homepage
 

amundsengremlin @ cc7e9b20

+Subproject commit cc7e9b2005425acbec5350c5627a43698d624805