Add files from wiki entry on extending netbox image with plugins to gitignore

[ryanmerolle]

Desired Behavior

Really just helps minimize confusion or friction for people.

.gitignore

plugin_requirements.txt
Dockerfile-Plugins

Contrast to Current Behavior

not in the gitignore.

Required Changes

No response

Discussion: Benefits and Drawbacks

No response

[tobiasge]

This would make adding those files into your own fork more complicated. I guess this is a question of what is the more common use case.