added spring data reference (#3388)

* added spring data reference

* satisfy linter

Co-authored-by: Eric Hellman <eric@hellman.net>

free-programming-books.md

@@ -79,6 +79,7 @@
   * [JasperReports](#jasperreports)
   * [Spring](#spring)
   * [Spring Boot](#spring-boot)
+  * [Spring Data](#spring-data)
   * [Spring Security](#spring-security)
   * [Wicket](#wicket)
 * [JavaScript](#javascript)
@@ -1527,6 +1528,11 @@ Kerridge (PDF) (email address *requested*, not required)
 * [Spring Boot Reference Guide](https://docs.spring.io/spring-boot/docs/current/reference/html/) - Phillip Webb et al. ([PDF](https://docs.spring.io/spring-boot/docs/current/reference/pdf/spring-boot-reference.pdf))
 
 
+#### Spring Data
+
+* [Spring Data Reference](https://docs.spring.io/spring-data/jpa/docs/current/reference/html) - Oliver Gierke, Thomas Darimont, Christoph Strobl, Mark Paluch, Jay Bryant
+
+
 #### Spring Security
 
 * [Spring Security Reference](http://docs.spring.io/spring-security/site/docs/current/reference/htmlsingle/) - Ben Alex, Luke Taylor, Rob Winch