Add usage label do Dockerfile

Dockerfile

@@ -1,5 +1,7 @@
 FROM node:latest
 
+LABEL org.label-schema.usage="https://code.r3pek.org/r3pek/docker-haraka/raw/branch/master/README.md"
+
 ENV HOSTNAME=${HOSTNAME:-haraka.localdomain}
 ENV EMAIL=${EMAIL:-test@email.tld}
 ENV USE_LETSENCRYPT=${USE_LETSENCRYPT:-false}