add link to CPPZMQ tour to README

[egpbos]

Problem: a comprehensive documentation of cppzmq is missing from this repo.

Solution: add a link to the very extensive tour of cppzmq by @brettviren.

[coveralls]

Pull Request Test Coverage Report for Build 1380637239

• 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 86.037%

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Totals
Change from base Build 1372505280:	0.0%
Covered Lines:	801
Relevant Lines:	931

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💛 - Coveralls

[gummif]

Thanks for that, that is way better than no documentation, even if written as a learning exercice.