Slack notification: {{.Params.buildStatus}} substitution not working

EDIT: found out that this param was missing in the YAML config file that was supposed to provide it. So then it's just the first question that remains: where to find a list of all variables that can be used?

**1st thing:**
It would be really great to have a list of all available variables for the notification files.

**2nd thing:**
I have used this text inside slack.json notification file (taken from [this example file](https://github.com/GoogleCloudPlatform/cloud-build-notifiers/blob/03512548a658bb024d9d12e6e7ae872d051ab1a2/slack/slack.json#L6)):
`"Cloud Build in {{.Build.ProjectId}} for branch {{.Build.Substitutions.BRANCH_NAME}} finshed with status *{{.Params.buildStatus}}*"`
and it resulted into
`... with status **`

Any idea why this behavior?
Also - do you have any link to display all possible variables? It could go into Readme file.