better explain content of bids app tables

[Remi-Gau]

Could you also clarify what “status unstable” means on the table "Status of the default branch on the source repository"?

ah yeah that's underexplained at the moment: we should add some info

so the status of the latest image should mean: if I pull the latest 'stable' version from dockerhub and run the test contained in the repo: do they pass?

the unstable table is meant to represent the status of the bids-app on the default branch of the repo. with every commit or pull request merge we run the following tests:

• try to build the docker image
• try to use that image to run the bids app on some lightweight bids dataset
• if that passes we push that image to dockerhub with an 'unstable' tag (can be useful if users want to check the bleeding edge version of an app that has not yet been released)
• if any of those fail then this table should let us know about it

as you can see this is still work in progress

Originally posted by @Remi-Gau in #560