Add How To documentation section for 'Play Selection' feature released in v1.6.0

[brandongregoryscott]

This feature has been available since v1.6.0, but it might not be obvious to new users that it is available - it makes listening to one section or part of a section over faster than listening to the whole track again! (Or even starting from a specific point in the track)

https://github.com/brandongregoryscott/beets/releases/tag/v1.6.0