Add Documentation - How to expose input in config file?

[dgarijo]

We don't specify how to do this. We do it for parameters, but inputs can also be declared in config files. Therefore we should follow a similar approach in case this happens. Otherwise mic will look for exactly the same file, and it will fail

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[dgarijo]

This can be done the same way parameters are done. But it's not documented!!

TO DO: Add in the documentation that this is possible