Updated README explaining locale in Test Plans

[arpavan]

Description

Updated existing documentation explaining the addition of locale to Test Plans -

1. Added a doc explaining the locale field to the testSettings README.
2. Updated the general README with comments explaining locale specific usage, and specific comments about the new samples added.

Checklist

• [N/A] The code change is covered by unit tests. I have added tests that prove my fix is effective or that my feature works
• [N/A] I have performed end-to-end test locally.
• [N/A] New and existing unit tests pass locally with my changes
• [N/A] I have commented my code, particularly in hard-to-understand areas
• [X] I have made corresponding changes to the documentation
• [N/A] My changes generate no new warnings
• [X] I used clear names for everything
• [X] I have performed a self-review of my own code

[github-actions[bot]]

Package	Line Rate	Branch Rate	Complexity	Health
Microsoft.PowerApps.TestEngine	93%	89%	800	✔
Summary	93% (2171 / 2330)	89% (490 / 548)	800	✔

Minimum allowed line rate is 50%