[Docs] Add import/recommended ruleset to README

[edemaine]

Here's a simple fix to #2032. As suggested, I didn't increase the prominence of import/recommended any more than import/errors and import/warnings already had. Let me know if you'd like more or less change.

[coveralls]

Coverage increased (+1.6%) to 71.142% when pulling 747772e on edemaine:import-recommended into e871a9a on benmosher:master.

[coveralls]

Coverage increased (+1.0%) to 82.358% when pulling 7c1e8e4 on edemaine:import-recommended into a943fd0 on benmosher:master.

[coveralls]

Coverage increased (+1.6%) to 71.142% when pulling 747772e on edemaine:import-recommended into e871a9a on benmosher:master.

[coveralls]

Coverage increased (+1.6%) to 71.142% when pulling 747772e on edemaine:import-recommended into e871a9a on benmosher:master.