chore(apollo): Add details about authorization modes to Apollo AppSync README

[mattcreaser]

• PR title and description conform to Pull Request guidelines.

Issue #, if available:

Description of changes:
Adds some additional details around authorization modes to the apollo/README file.

How did you test these changes?
n/a

Documentation update required?

• No
• Yes (Please include a PR link for the documentation update)

General Checklist

• Added Unit Tests
• Added Integration Tests
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 44.03%. Comparing base (7f874ed) to head (b493020).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2912      +/-   ##
==========================================
+ Coverage   44.01%   44.03%   +0.01%     
==========================================
  Files         926      931       +5     
  Lines       30485    30688     +203     
  Branches     4351     4496     +145     
==========================================
+ Hits        13419    13512      +93     
- Misses      15601    15693      +92     
- Partials     1465     1483      +18