Docs: Add example using JOIN / Db.read

[seangwright]

• Fixes a missing WHERE clause when params are used
• Adds an example using SQL JOIN and Db.read

I recently tried using Donald but didn't see any examples using Db.read when there's a one-to-many relationship using a JOIN.

[IanWitham]

Yikes. I think I'd rather just post-process the results with List.GroupBy than deal with this. I wonder if there's some official guidance on how to do joins in Donald though?