Summary section for archiving policy

[stedolan]

While it is precise and detailed, I found the new archiving policy a bit hard to read as a package maintainer when @jmid linked it on a recent package upload PR of mine. This patch adds a summary to the top which will hopefully be helpful to others who come across this policy for the first time.

I'm not proposing any changes to the policy itself: the summary is based on my understanding of the text below, and (I hope) doesn't contradict it anywhere, even if it doesn't spell out all details up front.

Maintenance note: the summary also includes the current compiler cutoff threshold, so if this PR is accepted there will be two locations in the document that need to be updated when the threshold changes.

cc @hannesm

[hannesm]

Thanks for your summary. I have two comments:

• talking about "package version" -- the x-maintenance-intent is taken from the latest version of each package (so if you have foo.0.1.0 and foo.0.2.0, the x-maintenance-intent is taken from 0.2.0 -- so you don't need to have this metadata in each version)
• if there's no x-maintenance-intent specified, the default (for now!?) is "any" -- so if you don't specify any, all versions will be kept

[stedolan]

Thanks for the clarifications, I've updated the summary

[jmid]

I think it is great to add this summary up front!
It will help when pointing newcomers here with an encouragement to add an x-maintenance-intent entry.