Documentation of --bids-filter-file input

[utooley]

It took me a little more work than necessary to figure out how to write the config.json file for --bids-filter-file properly (see Neurostars post, and eventually found an example in #2018 discussion. I'd was going to open a PR to put a link to an example config .json (in addition to the link to the specification in pybids) in the docs, but wasn't sure where such a file should live, given that it's a pybids thing? Or whether it could just go in-line in the Usage Notes for --bids-filter-file?

Example:

    "t1w": {
        "datatype": "anat",
        "session": "03",
        "acquisition": null,
        "suffix": "T1w"
    },
    "bold": {
        "datatype": "func",
        "session": "03",
        "suffix": "bold"
    }
}```