[improvement] Wrote a convention for white space (5m)

This is extracted from concordia-publishing-house/ledger#399.

It adds these conventions:

 * In general, leave one line break between methods
 * Multiple line breaks can be used to separate "paragraphs" of related methods (note the use of the word 'can': this allows convention allows this technique but doesn't require it)
 * There is interior no line break within a `context` or `describe` block