Remove reference to the mailing list from CONTRIBUTING.md

[stevenaw]

We mention "helping people on the mailing list" quite high up in https://github.com/nunit/nunit/blob/master/CONTRIBUTING.md

The mailing list was closed. We should consider removing the link as no new content or questions to answer will be posted.

[mikkelbu]

Perhaps we should replace the mailing list with a link to the Slack channel (although I'm rarely there 😄)

[OsirisTerje]

I am on the slack channel all the time, so that is fine by me.
The mailing list on google, however, I am never there.
And @rprouse closed it in Feb.

[stevenaw]

Great point about directing people to Slack instead!