documentation for stored procedures

[wibeasley]

@zozlak, To advertise the the package's breadth, consider adding an example of a stored procedure in RODBCext::sqlExecute() (ie, in the vignette and/or in the reference manual).

This isn't critical, because there are examples in SO. I just mean the user has to do additional searching (and might get distracted/discouraged along the way).

• http://stackoverflow.com/a/42426257/1082435
• http://stackoverflow.com/questions/30898712/using-r-with-rodbcext-and-rodbc-to-execute-a-sql-stored-procedure