Update map.csv and document

I have updated map.csv again.  I also added documentation on the process I use.  

In addition I have modified the query that generates the employer table to use outer joins so that if some entity is not in the mapping table it will still be included in the aggregations.
If you put this in a

\`\`\`sql
/* inside here */
\`\`\`

block then you'll get github syntax highlighting!