HOWTO new types of "contribution"

In light of the [ongoing discussion in BIDS](https://github.com/bids-standard/bids-specification/issues/627#issuecomment-829445877) I thought that tributors could be a nice fit! For that to happen we would need
- be able to create additional contribution categories, such as "steering-committee" or "bep032" to annotate contributors involved in a particular activity/sub-project. It seems that all-contributors already supports providing additional "contribution" types: https://allcontributors.org/docs/en/bot/configuration which is great!
- ideally, we should get a mechanism to "sync" individuals with github Teams. In the simplest case one way it could be github -> tributors direction:
- e.g. if someone is added to the team among https://github.com/orgs/bids-standard/teams , tributors
- possibly initiates configuration item for allcontributors
- adds that category in .all-contributors for that person
- and if removed from the team
- removes that category in .all-contributors for that person (related but another issue will follow)