Updated qc demo to include HWE calculations

Updated QC-demo.md and HWE query to include calculations for expected values, chi squared value, and a boolean for p-value significance.
Great comments in this query!

This query could be a little more succinct with one more level of nesting for the select (e.g., an inner select computes the values needed by the chi-square calc in the outer select)  See this [example](https://github.com/googlegenomics/bigquery-examples/tree/master/1000genomes/data-stories/reproducing-hardy-weinberg-equilibrium).

But this is fine as-is though, so feel free to disregard this comment!