Procfile: add a comment on how to use this

Procfile

@@ -1,3 +1,4 @@
+# Use goreman to run `go get github.com/mattn/goreman`
 etcd0: ./etcd -id 0x0 -l :8080 -peers '0x0=localhost:8080&0x1=localhost:8081&0x2=localhost:8082'
 etcd1: ./etcd -id 0x1 -l :8081 -peers '0x0=localhost:8080&0x1=localhost:8081&0x2=localhost:8082'
 etcd2: ./etcd -id 0x2 -l :8082 -peers '0x0=localhost:8080&0x1=localhost:8081&0x2=localhost:8082'