Add definition, support content, links to README

[EoinShaughnessy]

Fixes #23 and #562.

This PR updates the country-and-territory-autocomplete README with:

• a definition of the component
• advice to users about when not to use it
• a link to the accessible-autocomplete README
• information about what support we can provide
• links to pinned support and research issues

[EoinShaughnessy]

We also need to replace the placeholder links with the real links for the research and support issues.

[EoinShaughnessy]

@lfdebrux I made a mistake when I tried to rebase. If it's ok, can I get your help sometime getting back to where I was before rebasing?

[EoinShaughnessy]

The 'Reopen pull request' button below is greyed-out, so I guess that's not an option...is it?

[lfdebrux]

@EoinShaughnessy Ah, I think when you rebased you may have ended up dropping your changes by accident, which made GitHub think that the PR had no new changes and had already been merged! 😕 No worries, we can sort it, although we might have to open a new PR.

Getting back to the old state is fairly easy, GitHub helpfully tells you now what the commit id was before a rebase was done:

So to reset things to the way they were before you can use the commands

git checkout update-readme
git reset --hard 5a6fb33
git push --force-with-lease origin update-readme

[EoinShaughnessy]

@calvin-lau-sig7 Can I get a final review? :)