Fixed FreeBSD-Part of README (#33)

* add building prerequisites for FreeBSD

* FreeBSD: removed instruction to install SRC-Package

Installation of 'net-snmp' is sufficient

* remove "$" for compat. reasons

Author: mrdsam

README.md

@@ -11,7 +11,7 @@ This plugin uses the bulk-get mode of SNMPv2 by default - pass it the option -m
 In order to compile this plugin you need the `NET SNMP Development` package
 (`libsnmp-dev` for Debian) as well as `autoconf` and the standard compilation tools.
 
-Example for Debian:
+#### Example for Debian:
 
 ```
 apt-get update
@@ -41,6 +41,20 @@ make
 make install
 ```
 
+#### Prerequisites for FreeBSD
+
+```
+sudo pkg install automake gcc net-snmp
+```
+
+To build:
+```
+autoreconf
+./configure --prefix=/usr/local   ## works, but you should have your own plugins-dir
+make
+sudo make install   ## optional
+```
+
 ### Usage