Update readme regarding mysql (#54)

Co-authored-by: Michael Voříšek <[email protected]>

Author: marios88

README.md

@@ -342,7 +342,7 @@ $mutex->synchronized(function () use ($pdo, $accountId, $amount) {
 #### MySQLMutex
 
 The **MySQLMutex** uses MySQL's
-[`GET_LOCK`](https://dev.mysql.com/doc/refman/8.0/en/miscellaneous-functions.html#function_get-lock)
+[`GET_LOCK`](https://dev.mysql.com/doc/refman/9.0/en/locking-functions.html#function_get-lock)
 function.
 
 It supports time outs. If the connection to the database server is lost or
@@ -352,6 +352,9 @@ Note that before MySQL 5.7.5 you cannot use nested locks, any new lock will
 silently release already held locks. You should probably refrain from using this
 mutex on MySQL versions < 5.7.5.
 
+Also note that `GET_LOCK` function is server wide and the MySQL manual suggests
+you to namespace your locks like `dbname.lockname`.
+
 ```php
 $pdo = new PDO("mysql:host=localhost;dbname=test", "username");