Clarify configuration in README

[stephen-soltesz]

This change adds new configuration hints based on feedback from https://github.com/m-lab/alertmanager-github-receiver/issues/50

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 149

• 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 100.0%

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Totals
Change from base Build 147:	0.0%
Covered Lines:	408
Relevant Lines:	408

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💛 - Coveralls