Add relevant info from "Optimizing Qt Application" blog post to Best Practices documentation

Not everything may be relevant, and some is probably covered by the performance section, but there seems to be a bunch of stuff that we could use. See the "Optimizing Your Application: ‘Do’s and ‘Do not’s!" section:

http://blog.qt.io/blog/2016/04/27/fast-booting-qt-devices-part-2-optimizing-qt-application/