Add BSD license type to the usage in readme

The license was added in #2

Author: x1ddos

README.md

@@ -15,7 +15,7 @@ to any file that already has one.
     addlicense [flags] pattern [pattern ...]
 
     -c copyright holder (default "Google Inc.")
-    -l license type: apache, mit (default "apache")
+    -l license type: apache, bsd, mit (default "apache")
     -y year (default 2016)
 
 The pattern argument can be provided multiple times, and may also refer