Create encoding "guidelines"

[ahankinson]

While the version 2 specification will help clarify the boundaries of the Plaine & Easie code, it will also create a hole. Previous versions were part specification, part encoding guideline. In Version 2 the guidelines are largely going to feature less in the specification, but there is likely still an audience for them.

So we should create a second document that can contain implementation guides, best practices, or other guidelines. Like "record transposing instruments in a non-transposed way" or "Should contain 3-4 measures", etc.