Add a link to the BetterCrypto reference

dovecot/conf/dovecot.conf

@@ -57,6 +57,8 @@ namespace inbox {
 ssl = yes
 ssl_cert = </certs/cert.pem
 ssl_key = </certs/key.pem
+# TLS hardening is based on the following documentation:
+# https://bettercrypto.org/static/applied-crypto-hardening.pdf
 ssl_protocols=!SSLv3 !SSLv2
 ssl_cipher_list = EDH+CAMELLIA:EDH+aRSA:EECDH+aRSA+AESGCM:EECDH+aRSA+SHA256:EECDH:+CAMELLIA128:+AES128:+SSLv3:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!DSS:!RC4:!SEED:!IDEA:!ECDSA:kEDH:CAMELLIA128-SHA:AES128-SHA
 ssl_prefer_server_ciphers = yes