[Nexus] GitHub workflow

[Pete Jemian]

Last year, we decided to adopt a GitHub workflow for the definitions 
repository where working changes would be contributed to a "development" 
branch.  On review, all changes posted to "development" would be merged 
into the "master" branch".

The official NeXus manual is published to the nexusformat.org web site 
from the version in the "master" branch when there are new changesets.

Consistently, we have been posting changes directly to "master", 
forgetting about the workflow decision described above.  This indicates 
that we need a different workflow where working changes are posted to 
"master".  Some other GitHub projects have adopted this same workflow. 
To identify releases, point releases are posted to a "release" branch or 
they are tagged.

I suggest we consider dropping the "development" branch and adding a 
"releases" branch for reviewed point releases of the definitions 
repository.  This will better match the way we are contributing.

Pete