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
1 file changed