Locally linked plugins

[mlynch]

Verify npm linking plugin works and explain this in plugin dev guide

[jcesarmobile]

At the moment linked plugins are not detected because we only read the dependencies from package.json.
Do we want to detect the linked plugins somehow? or document that they have to add the plugin name inside the dependencies of the package.json

[imhoffd]

I believe this might be fixed by this PR: #3040

[dotNetkow]

Documented this in the Capacitor docs.