[WELCOME] fix newb message that was not shown since the 'Guest' default group was added to the admin plugin
[WELCOME plugin] Make the Welcome plugin config file easier to understand for users. Old config structure is still valid ; no change required when upgrading.
[DOC] add documentation for the welcome plugin
[WELCOME plugin] update version
Merge pull request #67 from thomasleveil/#19-welcome-plugin

Closes #19 - simplify the welcome plugin config file