Add archiving annoucement in README

[jchannon]

This links to #3010

This adds a big banner in the README stating Nancy is no longer under development.

When this PR is merged the following should take place:

• [ ] All repositories will be archived and made readonly
• [ ] Announcement on twitter pointing to announcement
• [ ] Announcement to @channel in Slack.

[jchannon]

I wanted to get a wee dig in but maybe it's too obvious :)

On Mon, 6 Apr 2020 at 18:44, Damian Hickey notifications@github.com wrote:

@damianh commented on this pull request.

In README.md https://github.com/NancyFx/Nancy/pull/3011#discussion_r404274437:

@@ -1,3 +1,21 @@ +# Announcement - Nancy is no longer being maintained! + +We would like to thank all the thousands of users of Nancy, all the people who wrote blog posts, conference speakers, video producers and those that spread the word of Nancy. + +We would like to thank the 150+ contributors to Nancy that made it what it became, without you the work would have been much harder and opportunities missed. + +We would like to thank VQ for being one of the few companies that paid for our open source efforts.

This feel like a wee dig to others. Perhaps this? "We would like to thank VQ http://www.vqcomms.com for sponsoring for our open source efforts. "

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