Add spec and doc for class methods

[AlexWayfer]

Without library changes: it seems working.

[coveralls]

Pull Request Test Coverage Report for Build 7

• 12 of 12 (100.0%) changed or added relevant lines in 1 file 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 4:	0.0%
Covered Lines:	124
Relevant Lines:	124

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💛 - Coveralls

[tycooon]

Well, that's working just because singleton classes work the same way as regular ones in Ruby, but I think that having this in Readme (and in specs) is nice.

[AlexWayfer]

Well, that's working just because singleton classes work the same way as regular ones in Ruby

Yes, you're right!

[tycooon]

Thanks for the patch by the way 👍