updated links and info about SlickGrid.

[GerHobbelt]

6pac (Ben McIntyre) has shown to be a consistent and otherwise superb mainline maintainer of this important and beautiful grid over the years.

See also the Maintainer note I added in the slickgrid source file and the READMEs ato both original repo https://github.com/mleibman/SlickGrid and maintained continuation of that project at https://github.com/6pac/SlickGrid.

I think it's a pitty the JSpreadsheets lists SlickGrid work as dating back to 2016 while work continues to be done on it in 2019. Hence this edit and pull req.

[GerHobbelt]

Another thought here: The main repo still has the most stars (6K+) while the 6pac branch only has 1K+ which would push SlickGrid down the order of appearance here, which is not my intent.

What this pull-req does not fix is keeping the original the stars count of the original while the maintenance is happening in a fork due to particular circumstances where control has not been transferred on the main repo.

Maybe I should instead only augment the information in the original JSpreadsheets entry for slickgrid so that everyone is pointed to the maintained repo, which clearly points out the situation and the original repo links anyway...

🤔 🤔 🤔

Hm, maybe I should submit an alternative pullreq....