update readme

README.md

@@ -4,6 +4,11 @@ Python bindings for Sarah's `AHloc3d` (actually depends on this fork here: https
 
 # Instructions
 
+
+- Install the `cffi` package, e.g.
+```sh
+pip install --user cffi
+```
 - Clone the repo https://git.tpi.uni-jena.de/fatteneder/ahloc3d
 - Compile `AHloc3d` into a shared library (I think only dependency is `MPI` which is not used
   by `pyloc3d` but still needs to be installed)