Documentation: Creating Composer Packages

@kenjis I think the package developer should think ahead about the namespace name. So to speak, a test of maturity.

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@kenjis I placed the module in the vendor directory.
That is, if
"Acme\\" : "vendor/Acme/" - class autoloading works but short config name is not present and also Acme namespace is not visible via "php spark namespaces" command.
"Acme\\" : "Acme/" - everything works as expected.

Why did I specify vendor/Acme/?
If the developer can use the composer locally, but not on the server (that is, the developer stores the vendor directory in the repository and copies it to the server), then in this case he may well place his module in the vendor directory.
I believe that there should be no such restriction.

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@luckmoshy Yes, I may be biased. Like most of the core team. =)