Updated README.md with info on how-to test new releases.

[Hedda]

Updated README.md with info on how-to test new releases.

Copied this text from zigpy-cc so credits go to @sanyatuning

[coveralls]

Pull Request Test Coverage Report for Build 287

• 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 99.718%

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Totals
Change from base Build 282:	0.0%
Covered Lines:	708
Relevant Lines:	710

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💛 - Coveralls