changes: Add 'Changes' section to track changes since previous versions of the spec

This commit also includes updated HTML CSS.

Change-Id: I0f45e3fba25587f97cbb63332d457baacb515f40
diff --git a/index.rst b/index.rst
index a6ff962..cbe426d 100644
--- a/index.rst
+++ b/index.rst
@@ -12,6 +12,7 @@
    data
    signature
    types
+   changelog
    :maxdepth: 3
 
 
@@ -21,4 +22,3 @@
 .. * :ref:`genindex`
 .. * :ref:`modindex`
 .. * :ref:`search`
-