Add messages after installation

[MaiaDereva]

To help users, add 2 messages at the top of the admin panel just after installing the plugin. Example: "The plugin is now installed but need two important things to do before starting using it:

• Goto Settings>Options WP-LDP to initialize the containers
• Goto Settings>Permaliens to reinitialize the htaccess file"

[balessan]

Good enhancement proposal, thanks @MaiaDereva ;-)

[balessan]

A clean thing could be to use intro.js, like in the following article: https://www.sitepoint.com/creating-intro-js-powered-tours-wordpress/

But in a more plugin-related hardcoded way.