Add "How to Validate Counts and Cardinality" documentation.

[shawnbrown]

Add how-to documentation for checking counts and cardinality.

Should demonstrate:

• using len(data) to validate count of data elements
• using collections.Counter(data) to validate counts per value

Should also mention that cardinality is a descriptive statistic that can be calculated with many other tools that a developer might use (df[0].applymap(bool).sum(), select('A').filter().count(), etc.).