Add missing package-level comments

Author: stv0g

examples/dial/psk/main.go

@@ -1,3 +1,4 @@
+// Package main implements an example DTLS client using a pre-shared key.
 package main
 
 import (

examples/dial/selfsign/main.go

@@ -1,3 +1,4 @@
+// Package main implements a DTLS client using self-signed certificates.
 package main
 
 import (

examples/dial/verify/main.go

@@ -1,3 +1,4 @@
+// Package main implements a DTLS client using a client certificate.
 package main
 
 import (

examples/listen/psk/main.go

@@ -1,3 +1,4 @@
+// Package main implements a DTLS server using a pre-shared key.
 package main
 
 import (

examples/listen/selfsign/main.go

@@ -1,3 +1,4 @@
+// Package main implements an example DTLS server using self-signed certificates.
 package main
 
 import (

examples/listen/verify/main.go

@@ -1,3 +1,4 @@
+// Package main implements an example DTLS server which verifies client certificates.
 package main
 
 import (