tools: add remark-lint configuration in .remarkrc

Specifies the configuration for remark-lint, a markdown linter.

This configuration does not cause any warnings on any of the currently
present *.md files (ignoring thirdparty).

It is useful not only for possible future tooling that would check the
markdown files syntax, but also as a configuration for editor plugins,
e.g. linter-markdown for atom-linter.

Refs: https://github.com/nodejs/node/pull/7637
Refs: https://github.com/nodejs/node/pull/7727
Refs: https://github.com/nodejs/node/pull/7757
PR-URL: https://github.com/nodejs/node/pull/7729


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Robert Jefe Lindstaedt <robert.lindstaedt@gmail.com>