update README.md to reflect current maintenance status

[danc86]

Yes I wanted to ensure we continue pointing people at notmuch in case they find something more to their liking there -- it's certainly true that the notmuch ecosystem has much broader innovation happening whereas sup is not going to change much in future.

I considered leaving the link right near the top but I thought that would be doing sup a disservice. It is, after all, a complete and working mail client so there is no reason we should be scaring people away.

[IPv2]

I couldn't agree more with the rationale and decision. :-)

Also this reminded me: this might be a separate PR, but perhaps we should consider adding a note about how sup is being maintained, but isn't seeking new feature development? We have the mailing list email to this effect, but perhaps it should also be in the README.

[danc86]

That's a good idea. I was going to add a sentence about sup being "feature complete" to indicate that we're not looking to add any major new features... but then I forgot about that when I wrote this patch last night. :-) I'll update it.

[IPv2]

Sounds like a plan, thanks @danc86. :-) I have a handful of very minor unrelated tweaks for README.md, which I'll raise in a separate PR.

[danc86]

Amended to add some wording about no new feature development

[danc86]

Amended to add separate sections about "Maintenance status" and "Alternatives".

We could probably go back and forth on this one forever but at some point we should just call it good enough and ship a release :-)