updates to the Best Practice document

Following today's BP call, I've now added into the BP doc what I was
talking about:

* A section explaining about Coverages [1] (thanks to Jon Blower; I
repurposed one of his Melodies blog posts!)
* The beginnings of a section that tries to provide a linear path through
the decisions you might make when publishing data: "How to use these best
practices" [2] ... this tries to combine SDW and DWBP best practices into a
coherent whole ... that said, I've found it really hard to plan this out; I
think it's working (& there's more in my head that I unfortunately don't
have time to write before I disappear tomorrow ... leaving no more time for
update before TPAC.

Hope these additions are worthwhile.

Jeremy

[1]: http://w3c.github.io/sdw/bp/#coverages
[2]: http://w3c.github.io/sdw/bp/#how-to-use