doc: *.md formatting fixes in the top-level dir

Continuing what a58b48bc did for the
doc/ dir, this fixes some formatting issues in the *.md files that
are placed directly in the top-level directory.

README.md changes are excluded as they are covered by
https://github.com/nodejs/node/pull/7971

Refs: https://github.com/nodejs/node/pull/7637
PR-URL: https://github.com/nodejs/node/pull/7727


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>