Include OpenMP installation guide

README.md

 # N Body Simulation
 
 This code simulates particles under self-gravity. The forces are calculated via direct summation or with a tree and multipole expansion. 
+
 Most of the code is written in C++, Python is used to make plots and animations. OpenMP is used for parallelisation. 
 
+## Install OpenMP
+
+https://www.geeksforgeeks.org/openmp-introduction-with-installation-guide/
+
 ## Make build directory
 
 ```
@@ -11,9 +16,9 @@ nbody$ cd build
 nbody/build$ cmake ..
 ```
 
-## Compiling and running the simulation
+## Compiling and running the program
 
 ```
 nbody/build$ make
 nbody/build$ ./nbody
-```
\ No newline at end of file
+```