mcp3425: Mention python-smbus requirement

Author: dbrgn

README.md

@@ -69,6 +69,8 @@ The plugin assumes that you're using three voltage divider resistors to bring
 the voltage into a measurable range. You can configure them in the Python
 script.
 
+This plugin requires the python-smbus package to be installed.
+
 There are currently no configuration options available.
 
     LoadPlugin python

mcp3425.py

@@ -4,6 +4,8 @@
 
 Return supply voltage (in mV) from ADC readings.
 
+This plugin requires the python-smbus package to be installed.
+
 Configure your three voltage divider resistors below.
 Default values are 6.8K, 3.6K and 470.