docs: add upgrading section to index of documentation (#248)

* docs: adds UPGRADING.md, not to readme, to help inform users about migration to v2

* docs: erroneous version number

* Update UPGRADING.md

Co-authored-by: BenWhitehead <BenWhitehead@users.noreply.github.com>

* docs: clarify enums statement

* docs: add migration section to docs index

Co-authored-by: BenWhitehead <BenWhitehead@users.noreply.github.com>

Author: crwilcox

docs/index.rst

@@ -19,6 +19,15 @@ API Reference
   types
   admin_client
 
+Migration Guide
+---------------
+
+See the guide below for instructions on migrating to the 2.x release of this library.
+
+.. toctree::
+    :maxdepth: 2
+
+    UPGRADING
 
 Changelog
 ---------