Tutorial of customised group masks

[htwangtw]

The default behaviour of the app is to create a group mask per fMRIPrep derivative.
This might not be ideal for multisite or dataset with batches.
User will need to create there own group mask and apply it to all the derivatives.
To make this happen, two features are required

1. Allow user to point to existing working directory that contains the group mask
2. A set of instructions about how to generate the group mask using the existing utilities.

Suggestions welcomed!

[htwangtw]

Move this to backlog as this is a really hacked use case for this library, and we are encouraging people to use the container