Update README for autogenerated subdirectories

[chornge]

"You must never..." implies a prohibition where doing the opposite would lead to an undesired or unpredictable outcome. That isn't the case here since the changes made to autogenerated subdirectories get overwritten on code regeneration. Also, there's no indication that changes could/would affect SPM builds (which would justify the prohibition).

"You should not..." implies more of a recommendation than a prohibition since it is wasted effort because all changes will be overwritten in order for the package to remain buildable.

Checklist

• I've read the Contribution Guidelines.
• My contributions are licensed under the Swift license.
• I've followed the coding style for the rest of the project.
• I've updated the documentation if necessary.
• I've verified that my change does not break any existing tests.
• I've added tests covering all new code paths my change adds to the project (if appropriate).

[chornge]

ADDITIONAL:

In the default checklist template provided, two hyperlinks lead to error screens:
• The Contribution Guidelines hyperlink leads to a "Not Found" screen.
• The Swift license hyperlink leads to "This Page Isn't Working".

I replaced the error links with the following urls:
• https://github.com/apple/swift-atomics/blob/919eb1d83e02121cdb434c7bfc1f0c66ef17febe/CONTRIBUTING.md
• https://swift.org/LICENSE.txt