Be more explicit with how to get the Python modules.

Author: jclarke-csco

snmp_entity/README.md

@@ -25,5 +25,18 @@ This script has been tested with Python 2.7, and requires the following non-defa
 
 * net-snmp's netsnmp module (see http://net-snmp.sourceforge.net/wiki/index.php/Python_Bindings)
 
- NOTE: On Ubuntu and Debian, this package is python-netsnmp
- On CentOS, this is net-snmp-python
+ Ubuntu or Debian:
+
+ ```
+ $ sudo apt-get install python-netsnmp
+ ```
+
+ CentOS:
+
+ ```
+ $ yum install net-snmp-python
+ ```
+
+ FreeBSD:
+
+ Install `/usr/ports/net-mgmt/net-snmp` and make sure the PYTHON option is enabled.