Enhance documentation mysql

As it is unclear where I should post requests concerning documentation (github issues? Discourse?), i'm posting it here.

This is related to the follow up of Issue 857.

I would like to suggest to enhance the README.md of node-red-node-mysql by mentioning the mysqlConnectionLimit which can be added to settings.js in order to avoid too many parallel sessions and reaching the default limit on MySQL/MariaDB server side.

As a more general note, I would suggest that any use of the parameters within RED.settings. in the code source of any node to be mentioned in their attached README.

Note: perhaps this topic is used by people facing the issue, but I believe it is worth adding a line in the doc/README.

Documentation PR's are alway welcome