add dependencies to the readme

README.md

@@ -4,6 +4,10 @@ vdstatus
 A simple replacement for i3status that is both extensible and customizable.
 Documentation is pending, but no promises.
 
-Requirements:
+Base requirements:
 * python 3
-* pyyaml
+* [pyyaml](https://pyyaml.org/)
+
+Some plugins have dependencies of their own:
+* [psutil](https://github.com/giampaolo/psutil) - for mem.py and disk.py
+* [pytz](https://pythonhosted.org/pytz/) - optional for date.py (for custom time zone)