Update README with notice about Authentication Policies for OIE

[epierce]

Description

The okta-aws-cli project on GitHub notes that in OIE the OIDC native app should have equivalent policies to the AWS Account Federation app(s) or else more stringent policies on the AWS app(s) may result in a 400 Bad Request error:

https://github.com/okta/okta-aws-cli#recommendations

Related Issue

420

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [ ] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.