Add mysqldump flags docs link (#329)

Author: lelafa

README.md

@@ -437,7 +437,7 @@ wp db export [<file>] [--dbuser=<value>] [--dbpass=<value>] [--<field>=<value>]
 **Alias:** `dump`
 
 Runs `mysqldump` utility using `DB_HOST`, `DB_NAME`, `DB_USER` and
-`DB_PASSWORD` database credentials specified in wp-config.php. Accepts any valid `mysqldump` flags.
+`DB_PASSWORD` database credentials specified in wp-config.php. Accepts any valid [`mysqldump` flags](https://dev.mysql.com/doc/en/mysqldump.html#mysqldump-option-summary).
 
 **OPTIONS**
 

src/DB_Command.php

@@ -636,7 +636,7 @@ public function query( $args, $assoc_args ) {
 	 * Exports the database to a file or to STDOUT.
 	 *
 	 * Runs `mysqldump` utility using `DB_HOST`, `DB_NAME`, `DB_USER` and
-	 * `DB_PASSWORD` database credentials specified in wp-config.php. Accepts any valid `mysqldump` flags.
+	 * `DB_PASSWORD` database credentials specified in wp-config.php. Accepts any valid [`mysqldump` flags](https://dev.mysql.com/doc/en/mysqldump.html#mysqldump-option-summary).
 	 *
 	 * ## OPTIONS
 	 *