Added PR-description to README
## Introduction
-Copy this macaddr if you want to start a new
-plugin written in C.
+This plugin validates MAC-addresses. The following MAC-address-formats are allowed for now:
+
+ XX-XX-XX-XX-XX-XX
+ XX:XX:XX:XX:XX:XX
+ XXXXXX-XXXXXX
+
+If `set` is used, the value will be transformed to the format of the middle one.
+
+TODO:
+
+- Write tests
+- Update readme
+- Better error-messages
+- Maybe also support formats `XXXXXXXXXXXX` and `XXXX.XXXX.XXXX`
+
## Usage
