README: fix setup.py call, add prefix alternative

Thanks for the nice package @GregAshton , here's a minor suggested readme fix.

README.md

@@ -9,7 +9,11 @@ of coordinates), plot all possible 1D and 2D projections in the style of a
 ## Installation
 Clone the repository and then
 ```bash
-$ python setup install
+$ python setup.py install
+```
+or add a prefix to install in userspace, e.g.
+```bash
+$ python setup.py install --prefix=~/.local/
 ```
 
 ## Example