Update k8schain README -- don't recommend modifting authn.DefaultKeyc…

[imjasonh]

…hain

[codecov-commenter]

Codecov Report

Merging #1267 (92391a8) into main (cb47374) will not change coverage.
The diff coverage is n/a.

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  Hits         6116     6116           
  Misses       1571     1571           
  Partials      602      602           

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[imjasonh]

This questionable advice was discovered via kyverno/kyverno#3088 which resulted in ever-growing MultiKeychains, and memory leaks.

If this becomes a problem we might consider deprecating the mutable authn.DefaultKeychain and instead preferring a public method that returns an immutable Keychain impl.