Adds a .swagger-codegen-ignore file with instructions and examples.
The .swagger-codegen-ignore file is treated as a supporting file.
Every project will generate a .swagger-codegen-ignore file containing
instructions and examples.
This also adds support for 'common' files (defaults like
.swagger-codegen-ignore). In the case of the ignore file, a generator
may include a compiled template ignore file which outputs to the
outputDir folder as .swagger-codegen-ignore and the default file
generation will honor the already generated file.
The rules for .swagger-codegen-ignore are a simple subset of what you'd
find in .gitignore or .dockerignore. It supports recursive matching
(**), simple matching (*), matching files in the project root
(/filename), matching against directories (dir/), negation rules
(!previously/excluded/**/file).
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