Add some clarification on "Recommended versus Optional"

[mikeradka]

There is very often the question of how to distinguish between Recommended attributes and Optional attributes. Upon discussing this with some OCSF adopters, it seems one major distinction is that recommended empowers security teams to place more weight on deciding whether or not to log the attribute.

Adding some sentences to help emphasize this important distinction.

[floydtree]

Looks good, along with the .md file, we'll need to update the corresponding pdf file as well - Understanding OCSF.pdf