reference.conf: Document fingerprint generation

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doc/reference.conf

@@ -1318,6 +1318,9 @@ general {
 	/* certfp_method: the method that should be used for computing certificate fingerprints.
 	 * Acceptable options are sha1, sha256 and sha512.  Networks running versions of charybdis
 	 * prior to charybdis 3.5 MUST use sha1 for certfp_method.
+	 *
+	 * To generate a fingerprint from a certificate file, run the following:
+	 * $ openssl x509 -outform DER < your.crt | sha1sum     (or sha256sum, or sha512sum)
 	 */
 	certfp_method = sha1;
 };