Add docstring for DSI pydantic shim

dsi_pydantic_shim.py

@@ -1,3 +1,13 @@
+"""Shim to allow support for both Pydantic 1 and Pydantic 2.
+
+DSI must support both major versions of Pydantic because dbt-core depends on DSI. dbt-core users might be using an
+environment with either version, and we can't restrict them to one or the other. Here, we essentially import all
+Pydantic objects from version 1. Throughout the repo, we import those objects from this file instead of from Pydantic
+directly, meaning that we essentially only use Pydantic 1 in this repo, but without forcing that restriction on dbt
+users. The development environment for this repo should be pinned to Pydantic 1 to ensure devs get appropriate type
+hints.
+"""
+
 from importlib.metadata import version
 
 pydantic_version = version("pydantic")