Add note about maintenance status

I've decided to publish a fork of auryn and to maintain that myself, rather than the new AMPHP version. Since this repo is effectively unmaintained, I thought I'd offer my fork as a replacement whilst noting the AMPHP version as well. As such, I thought I'd offer a few notes to put in the readme of this repo explaining the situation.

I know that new projects should probably use AMPHP, but I was recently working on a large commercial project where that would be a reasonably large amount of work - and technical risk - and there was an appetite to keep using 'original' auryn with bugfixes.

Take it or leave it, of course :)