Adding newest OpenMP documentation (#3384)

* Adding OpenMP 5.0

* Adding PDF indicator

free-programming-books.md

@@ -1943,6 +1943,7 @@ Kerridge (PDF) (email address *requested*, not required)
 
 * [A Guide To OpenMP](http://bisqwit.iki.fi/story/howto/openmp/)
 * [OpenMP Application Programming Interface Standard Version 4.0](http://www.openmp.org/mp-documents/OpenMP4.0.0.pdf) (PDF)
+* [OpenMP Application Programming Interface Standard Version 5.0](https://www.openmp.org/wp-content/uploads/OpenMP-API-Specification-5.0.pdf) (PDF)
 
 
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