Document how to add or change a definition in the glossary

[vielmetti]

Pull requests welcome, but we will need a little bit of narrative for anyone for whom that's not a familiar phrase, and support from the project to accept edits or suggestions through issues.

In addition there should be a well-understood editor or editorial committee who accepts change requests.

[vielmetti]

In the "landscape" repo this was addressed with a custom template for issues.