Add AWS_SESSION_TOKEN example back to README

[humphd]

I had to use aws-session-token: ${{ secrets.AWS_SESSION_TOKEN }} today in order to authenticate successfully, and I only discovered it via this old commit 3042b00. It looks like it got removed in b8c74de#diff-b335630551682c19a781afebcf4d07bf978fb1f8ac04c6bf87428ed5106870f5L30, possibly by mistake?

I think it's pretty useful to include it, and I'd suggest adding it back.

[NathanielRN]

This is probably related to #302 , because the code to make it work with OIDC is merged as you pointed out in b8c74de#diff-b335630551682c19a781afebcf4d07bf978fb1f8ac04c6bf87428ed5106870f5L30 but the release for it has not been made yet.

[peterwoodworth]

There's now an example of this in the docs

[github-actions]

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