docs: Document GitHub Open ID Connect integration

[br3ndonland]

Issue #, if available:

#280
#281

Description of changes:

Thank you for the OpenID Connect ("OIDC") guidance in the README. However, I was confused when I read through it recently, because:

• There were no links to any documentation, either from AWS or GitHub
• The recommendation was added to the README before GitHub announced OIDC support

This PR will add links to the relevant documentation from both GitHub and AWS. I think it would point people in the right direction, if they're seeing this OIDC info here for the first time. Thanks for your consideration.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[tve]

Thanks for this PR, helped me implement OIDC! However, at first I got it badly wrong, granting all github users access to the AWS role! Specifically, the gihub docs have a little sentence stating "By default, the validation only includes the audience (aud) condition, so you must manually add a subject (sub) condition" which to me was just goobledygook, so I didn't configure an additional condition. Oops.
It might be helpful to:

• add a little phrase to "ensure you restrict access to the AWS role in IAM by specifying a condition in addition to the audience (aud) condition. Otherwise any github user can access the role"
• add a link: Claims sent by GitHub that can be checked in conditions
• add a link: Condition syntax and operators in AWS (took me a while to find that one)

[br3ndonland]

Great suggestions @tve, thanks! Your point about the subject condition in the assume role policy is a particularly important one. I've updated the PR accordingly.

[danielcompton]

One note for people following along, the @v1 tag currently doesn’t include OIDC support, even though the docs say to use @v1. You’ll either need to pin to a recent commit or use @master until they make a new release.

#271 (comment)

Update: I see that v1.6.0 was just released which includes OIDC support: https://github.com/aws-actions/configure-aws-credentials/blob/v1.6.0/CHANGELOG.md#160-2021-11-23

[peterwoodworth]

@Mergifyio update

[mergify]

update

✅ Branch has been successfully updated

[peterwoodworth]

This is an excellent improvement to our readme, thanks for your submission!