Add plugin list page

[ahmetb]

Introduces a lightweight plugins.md that can be linked in onboarding docs
at the "krew" repo as well as this "krew-index" repo.

Supersedes kubernetes-sigs/krew#303.

[corneliusweig]

I was thinking about how to include this in krew-index but avoid distributing to our users. One idea would be to use git hash-object -w and then reference the object in an annotated tag, which is pushed to github. Then only users pulling all tags would get the object. Junio Hamano does a similar trick with his public gpg key for signing tags in git/git. I still have to explore though, if github can show the markdown file, though.

On second thought: maybe an easier approach is to have a dedicated branch where the plugins.md lives. I'm quite sure this does work :)

[ahmetb]

The file is really small, like a plugin manifest. So I'm not too worried about it. It's like having a plugin that updates occassionally.

[corneliusweig]

One final thought before merging: as this file is not automatically updated, I think we should include a disclaimer on top that this may be outdated -- and say what revision this file refers to. WDYT?

[ahmetb]

Yeah that's possible, we should say run kubectl krew search for an up-to-date list. But that requires change in the tool, so we can do later.

[corneliusweig]

/lgtm
/approve

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