docs: invite feedback on the upstream choice and link the standing issue

The README said a change of upstream would be weighed in an issue here. That issue now exists: #3

This links it from the three places a reader meets the fork choice: the README's provenance paragraph, the site's "Which fork we build from" paragraph, and CONTRIBUTING's opening, which now says feedback needs no code. The ask in each is the same: a feature or fix in osscontainertools/kaniko that a consumer's builds need, named specifically.

Docs only. No pin changes; scripts/check-versions.sh passes.