Improve requirements in pkg/proxysql/README.md (#135)

* Improve pkg/proxysql/README.md

* Improve pkg/proxysql/README.md #2

Author: timvaillancourt

pkg/proxysql/README.md

@@ -14,8 +14,11 @@ All other statuses are considered unhealthy and therefore are ignored by freno,
 1. `OFFLINE_HARD` - a server that is completely offline
 
 ## Requirements
-1. The ProxySQL `--no-monitor` flag is not set
+1. The ProxySQL admin port is reachable to Freno
+1. The ProxySQL global variable [admin-stats_credentials](https://github.com/sysown/proxysql/wiki/Global-variables#admin-stats_credentials) is defined
+    - `ProxySQLUser` in `MySQLConfigurationSettings` (global) or `User` in `ProxySQLConfigurationSettings` (per-cluster) must be equal to `admin-stats_credentials`
+    - `ProxySQLPassword` in `MySQLConfigurationSettings` (global) or `Password` in `ProxySQLConfigurationSettings` (per-cluster) must be equal to `admin-stats_credentials`
 1. The [ProxySQL monitor module](https://github.com/sysown/proxysql/wiki/Monitor-Module) is enabled, eg: [`mysql-monitor_enabled`](https://github.com/sysown/proxysql/wiki/Global-variables#mysql-monitor_enabled) is `true`
+    - The ProxySQL `--no-monitor` daemon flag cannot be set
 1. The `max_replication_lag` column is defined for backend servers in [the `mysql_servers` admin table](https://github.com/sysown/proxysql/wiki/Main-(runtime)#mysql_servers)
-    - This ensures servers with lag do not receive reads but are still probed by freno
-
+    - This ensures reads do not receive stale data but lagging nodes are still probed