From @mrbobbytables's comment on Slack:

[add] a general blog warning msg - we do not go back and update anything more than a year old and the content may not reflect reality. ... like a template that checks the date, if more than a 1yr old add a preamble at the top

The comment was made in the context of the Dockershim-removal effort. I'm opening this PR following discussions with @celestehorgan.

The warning, which would appear only on posts that are >= 1 year old, might read something like this:

Warning: this blog post may contain outdated information, since posts that are more than a year old (like this one) are no longer updated.

Comments are welcome.

/cc @nate-double-u @sftim