Added a search engine book to subject list (#6124)

I added "Search Engines: Information Retrieval in Practice" by W. Bruce Croft, Donald Metzler, Trevor Strohman to the subject list.

books/free-programming-books-subjects.md

@@ -649,6 +649,7 @@ Kerridge (PDF) (email address *requested*, not required)
 ### Search Engines
 
 * [Elasticsearch: The Definitive Guide](https://www.elastic.co/guide/en/elasticsearch/guide/current/index.html) ([fork it on GH](https://github.com/elastic/elasticsearch-definitive-guide))
+* [Search Engines: Information Retrieval in Practice](https://ciir.cs.umass.edu/irbook) - W. Bruce Croft, Donald Metzler, Trevor Strohman (PDF)
 * [Solr for newbies workshop (2019)](https://github.com/hectorcorrea/solr-for-newbies) - Hector Correa ([PDF](https://github.com/hectorcorrea/solr-for-newbies/blob/master/tutorial.pdf))