Add topaz configuration document

[carabasdaniel]

Closes: #75

[coveralls]

Pull Request Test Coverage Report for Build 4811526512

• 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 38.188%

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Totals
Change from base Build 4810316457:	0.0%
Covered Lines:	1180
Relevant Lines:	3090

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💛 - Coveralls

[gertd]

I suggest we add an explicit paragraph on using environment variables inside the config file.

Right now, we only show using ${TOPAZ_DIR}, but it does not get explained in terms of behavior. For example, what happens when the env variable is not existing? It will return an empty string., effectively making it a NOP.