Add missing HtmlAttributes entry to README TOC

This was missed by 224ca38.

Change-Id: I0e046821e353a3dc9f9e8743cf7d162034eea5a7
Reviewed-on: http://gerrit.causes.com/42568
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>