Instruct MySQL users to set DATABASE_ENCODING to `utf8mb4`.

cf. GH-286

.env.example

@@ -26,6 +26,11 @@ DATABASE_PASSWORD=""
 #DATABASE_PORT=3306
 #DATABASE_SOCKET=/tmp/mysql.sock
 
+# MySQL only: If you are running a MySQL server >=5.5.3, you should
+# set DATABASE_ENCODING to utf8mb4 instead of utf8 so that the
+# database can hold 4-byte UTF-8 characters like emoji.
+#DATABASE_ENCODING=utf8mb4
+
 # ==== Additional required production settings ====
 
 # Configure Rails environment.  This should only be needed in production and may cause errors in development.