Add a performance note to the Readme.

[bobg]

Reddit user u/BombelHere correctly points out that decoupling is not always the right thing to do. This PR adds a section to the Readme explaining how programmers should think about when to decouple.

[github-actions]

Modver result

This report was generated by Modver,
a Go package and command that helps you obey semantic versioning rules in your Go module.

This PR does not require a change in your module’s version number.
(You might still consider bumping the patchlevel anyway.)

[coveralls]

Pull Request Test Coverage Report for Build 7963435786

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 79.146%

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Totals
Change from base Build 7931466127:	0.0%
Covered Lines:	649
Relevant Lines:	820

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💛 - Coveralls