Add a matrix multiplication tutorial

Add tutorial about writing of a high performance matrix multiplication
in C++ by Jean Guegant.

README.md

@@ -64,6 +64,7 @@ To get started, simply fork this repo. Please refer to [CONTRIBUTING.md](CONTRIB
   * [Part 7: Compound Statements](https://norasandler.com/2018/03/14/Write-a-Compiler-7.html)
   * [Part 8: Loops](https://norasandler.com/2018/04/10/Write-a-Compiler-8.html)
 * [Implementing a Language with LLVM](https://llvm.org/docs/tutorial/#kaleidoscope-implementing-a-language-with-llvm)
+* [High-Performance Matrix Multiplication](https://gist.github.com/nadavrot/5b35d44e8ba3dd718e595e40184d03f0)
 
 ### OpenGL: