Added brief installation instructions

Author: GijsVermarien

README.md

@@ -1,3 +1,7 @@
 # Hermite Post-Newtonian
 
 This code expands the original hermite code with post-newtonian terms.
+
+## Installation
+From the AMUSE source directory run the following git command `git submodule add [email protected]:amusecode/Hermite_GRX.git src/amuse/community/hermite_grx`. This will add the code as a submodule such that the respective git structures do not interfere. 
+The code can then be compiled using `make hermite_grx.code` or all codes can be built together using `python setup.py develop_build`.