Update readme file to include domainless gMSA pathway explanation within prerequisites

[SovietFrontier]

Issue #, if available:
N/A

Description of changes:
Update readme file to include domainless gMSA pathway explanation within prerequisites. In order to use credential fetcher with Amazon Elastic Container Service (Amazon ECS), you must use domainless gMSA as described in the documentation. Looking between that documentation and the information listed in the prerequisites here, it is conflicting. This update will add the caveat for using Amazon ECS - Fargate.

Testing done:
N/A

Merge Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your pull request.

General

• I have read the CONTRIBUTING doc
• I used the commit message format described in CONTRIBUTING
• I have updated any necessary documentation, including READMEs and comments (where appropriate)

Tests

• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have checked that my tests are not configured for a specific environment
• No tests as this is a documentation change.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[Beau-Gosse-dev]

Hi! Thanks for finding and fixing this issue! We'll get this reviewed and merged soon.