Add a description table for SQL keywords (checkout)

[sbhave77]

It would be useful to provide a table (in markdown for example) which provides descriptions about how each SQL query works (as a cheat sheet) for the learners. This will decrease cognitive burden due to having to remember all of these keywords and their use cases. Something like this (although the wording could use some work):

SQL Keyword	Description
SELECT	specifies which columns of the table are being selected
FROM	specifies which table is being selected from
WHERE	allows for specification of filter conditions
GROUP BY	specifies which columns to group the table by for aggregation. Warning: All columns not specified in group by must be aggregated otherwise you will get an error.