update readme environment set up.

README.md

@@ -32,6 +32,10 @@ Details of an anaconda environment is provided in the repository with the requir
 
         conda env create -f environment.yml
 
+Note that if you are on a mac then you need to use this command instead:
+
+        conda env create -f macenvironment.yml
+
 This creates and environment callled 'resultsanalysis' which can be loaded using:
 
         conda activate resultsanalysis