Add OpenMP reference card (#7731)

* Add OpenMP reference card

* moved OpenMP cheat sheet to C++

more/free-programming-cheatsheets.md

@@ -71,6 +71,7 @@
 * [C++ Cheatsheet](https://www.codewithharry.com/blogpost/cpp-cheatsheet) - CodeWithHarry (HTML)
 * [C++ Quick Reference](http://www.hoomanb.com/cs/quickref/CppQuickRef.pdf) - Hooman Baradaran (PDF)
 * [MPI Cheat Sheet](https://wtpc.github.io/clases/2018/mpicheatsheet.pdf) - SC Education (PDF)
+* [OpenMP 4.0 API C/C++ Syntax Quick Reference Card](https://www.openmp.org/wp-content/uploads/OpenMP-4.0-C.pdf) (PDF)
 
 
 ### Clojure