readme: Use <pre> and <sup> instead of ``` and <small>

* Gitblit does not support ```. Both Gitblit and GitHub support
  <pre lang>.

* GitHub Flavored Markdown doesn't support <small>.
  Gitblit only supports <small> inside paragraphs (not list items).
  Use <sup> instead.

* Drop <small> for note after json2.js (parentheses suffice).

Change-Id: I4abc6056b1b921b880a323c9108b6a9622bed139