Create 5.8.md (#5902)

* Create 5.8.md

Author: elsh

Documentation/ReleaseNotes/5.8.md

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+# SwiftPM 5.8 Release Notes
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+* [#5728]
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+In packages that specify resources using tools version 5.8 or later, the generated resource bundle accessor will import `Foundation.Bundle` for its own implementation only. _Clients_ of such packages therefore no longer silently import `Foundation`, preventing inadvertent use of Foundation extensions to standard library APIs, which helps to avoid unexpected code size increases.