Add upgrade guide to the documentation

[skip ci]
diff --git a/docs/index.rst b/docs/index.rst
index 4e7a848..d236611 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -14,6 +14,7 @@
 
    intro
    changelog
+   upgrade
 
 .. toctree::
    :caption: The Basics