Copy this folder into Nebula's extensions directory to install:

  Windows:  %LOCALAPPDATA%\Nebula\User Data\extensions\hello-nebula\
  macOS:    ~/Library/Application Support/Nebula/User Data/extensions/hello-nebula/
  Linux:    ~/.local/share/Nebula/User Data/extensions/hello-nebula/
            (or $XDG_DATA_HOME/Nebula/User Data/extensions/hello-nebula/)

Or open Settings > Plugins — the install path is shown there.

Then click Reload Plugins (or restart Nebula).

manifest.json fields:
  id, name, version, description, enabled
  content_scripts[].matches  (e.g. "*://*/*", "*://*.youtube.com/*", "<all_urls>")
  content_scripts[].js / css (paths relative to this folder)