Add Scout to the list of available backends (apple#337) * Add Scout to the list of available backends * Update README.md

Disable existential any build setting (apple#312) Motivation: On 5.8, if the experiment existential any setting is enabled and a consuming package holds an existential of a type from swift-log without an explicit 'any' then the build will fail. Modifications: - Disable the setting Result: Users of 5.8 aren't broken

Update README.md add shipbook as backend (apple#304) Motivation: Added shipbook as backend for the library. Modifications: Just added it to the backend list. Result: Will have a fuller list.

1.4.3