Added link to opensource SQL eBook (EbookFoundation#5387)

Author: bobbyiliev

books/free-programming-books.md

@@ -2848,6 +2848,7 @@ Kerridge (PDF) (email address *requested*, not required)
 ### SQL (implementation agnostic)
 
 * [Developing Time-Oriented Database Applications in SQL](https://www2.cs.arizona.edu/~rts/tdbbook.pdf) - Richard T. Snodgrass (PDF)
+* [Introduction to SQL](https://github.com/bobbyiliev/introduction-to-sql) - Bobby Iliev (Markdown, PDF)
 * [SQL For Web Nerds](http://philip.greenspun.com/sql/)
 * [SQL Notes for Professionals](http://goalkicker.com/SQLBook/) - Compiled from StackOverflow Documentation (PDF)
 * [SQL Queries Succinctly](https://www.syncfusion.com/ebooks/sql_queries_succinctly) - Nick Harrison