Update README to clarify that this repository is unmaintained

[schmod]

Currently, the GitHub Issue Template for this repository states:

we don't plan to merge pull requests or release future versions of Classic unless it's to solve an critical vulnerability report (which is unlikely).

This appears to be the only place where Yarn 1's deprecation is noted – at that, the notice is placed in in a HTML comment, which renders it invisible!).

Neither README.md nor the pull-request template note that this repository is unmaintained.

Before others waste hours contributing fixes and pull-requests, can we please put this notice in a more prominent place so that contributors (such as myself) do not waste their time fixing bugs or creating pull-requests?

Can a maintainer please update README.md to clarify that the Yarn 1.x release line is unmaintained, and will not receive fixes for non-security-related issues?

Also, the link to the 'Contributing' guide is broken.

[alanrps]

You are greek god, thank you @schmod! I was also wasting time.

[merceyz]

The "about" section has stated this for a while but we updated the README in 0a121f6 so closing this