make github username explcit

[Remi-Gau]

Using a tributors file to track BIDS contributors.

https://github.com/bids-standard/bids-specification/blob/master/.tributors

Several of them do not have a a github account.

Therefore I have been using a key name_familyname in the file.

But now I have no explicit way of knowing if that person has a github account.

Added it for some people: https://github.com/bids-standard/bids-specification/blob/790c0fbf72c01c0723f9afe3486cc154e0669d07/.tributors#L369

Is there an official way to handle that explicitly in tributors files?

[yarikoptic]

Frankly, I don't know the answer... most likely there is an implicit association of the record key with github, and indeed we do not have explicit "github" field, which is worth adding based on your use case