Document differences from findbugs-slf4j

[mattnelson]

The readme calls out

This provides almost same feature with findbugs-slf4j.

I am current using findbugs-slf4j and am wondering if the differences are only the patterns? Will the missing findbugs patterns be ported?

errorprone	findbugs
Slf4jPlaceholderMismatch	SLF4J_PLACE_HOLDER_MISMATCH
Slf4jFormatShouldBeConst	SLF4J_FORMAT_SHOULD_BE_CONST
	SLF4J_UNKNOWN_ARRAY
Slf4jLoggerShouldBePrivate	SLF4J_LOGGER_SHOULD_BE_PRIVATE
Slf4jLoggerShouldBeFinal	SLF4J_LOGGER_SHOULD_BE_FINAL
Slf4jLoggerShouldBeNonStatic	SLF4J_LOGGER_SHOULD_BE_NON_STATIC
Slf4jIllegalPassedClass	SLF4J_ILLEGAL_PASSED_CLASS
Slf4jSignOnlyFormat	SLF4J_SIGN_ONLY_FORMAT
	SLF4J_MANUALLY_PROVIDED_MESSAGE

https://github.com/KengoTODA/errorprone-slf4j#supported-bug-patterns
https://github.com/KengoTODA/findbugs-slf4j#provided-bug-patterns

[KengoTODA]

SLF4J_MANUALLY_PROVIDED_MESSAGE has been implemented in the v0.14.0 release.

I'll reconsider we really need SLF4J_UNKNOWN_ARRAY for errorprone or not.