markdownlint: allow sub/sup HTML

Github markdown only supports subscript and superscript with the
`<sub>` and `<sup>` HTML tags.  Allow those to be in our markdown files.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: If2ea93cd71e6da2f11ee04caed01321de5894453
diff --git a/config/markdownlint.yaml b/config/markdownlint.yaml
index 8783ec6..bcc4833 100644
--- a/config/markdownlint.yaml
+++ b/config/markdownlint.yaml
@@ -3,4 +3,4 @@
 MD024:
     siblings_only: true
 MD033:
-    allowed_elements: ["br"]
+    allowed_elements: ["br", "sub", "sup"]