Add registry-secrets help msg for build and deploy commands

Change-type: patch
Signed-off-by: Paulo Castro <paulo@balena.io>

lib/utils/messages.ts

@@ -19,3 +19,18 @@ export const balenaAsciiArt = `\
 | |_) | (_) || ||  ___/| | | || (_) |
 |_.__/ \\__,_||_| \\____/|_| |_| \\__,_|
 `;
+
+export const registrySecretsHelp = `\
+The --registry-secrets option specifies a JSON or YAML file containing private
+Docker registry usernames and passwords to be used when pulling base images.
+Sample registry-secrets YAML file:
+
+	'my-registry-server.com:25000':
+		username: ann
+		password: hunter2
+	'':  # Use the empty string to refer to the Docker Hub
+		username: mike
+		password: cze14
+	'eu.gcr.io':  # Google Container Registry
+		username: '_json_key'
+		password: '{escaped contents of the GCR keyfile.json file}'`;