Add MPI Cheat Sheet (#7729)

* Add MPI Cheat Sheet

* Fix order

* Sperate MPI and OpenMP to different sections

* Move MPI cheat sheet to C++

more/free-programming-cheatsheets.md

@@ -70,6 +70,7 @@
 
 * [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)
 
 
 ### Clojure