README: linkable extensions

I'd like to be able to deep-link to any extension in the user guide.
The Nth time I wanted to link in some discussion to a specific pandoc syntax, I realized I can send a pull request and see what you think :-)

This is one approach, turning them into sections.  Another alternative I see would be:

    **Extension: `inline_code_attributes`{#inline_code_attributes}**

But that'd be repetitively repetitive in the source.

The main drawback of sections is non-uniformity — some'll appear as h3, some as h4.
Sections also bloat the TOC, but that's a feature — otherwise there is no way to discover the permalink (without inspecting the DOM).