Custom Plugins: Documentation is unclear on location and usage of custom plugins

Existing documentation for plugins looks like:

https://github.com/googleapis/release-please/blob/3bae6137f939a4db2df1d32436453d1ac2a41e9f/docs/manifest-releaser.md#plugin-usage

> To use a plugin in your manifest based release-please setup, simply add it to the array of the "plugins" key in your release-please-config.json. (Note: the plugin must already be implemented, see below)
>

What does `the plugin must already be implemented` ?

a) must exist in the release-please repo as google sanctioned code? or
b) can be a valid nodejs import string?

Ideally it should just be `b)` so i can do :

```
{
"plugins": [
{
"type": "@my-monorepo/release-please-nx-workspace"
}
]
}
```

This has useful benefit:

- some plugins are going to need constant updating, for example an nxdev workspace plugin that needs to be part of the release-please repo would need constant updating to stay relevant with the latest version of nxdev.
- some plugins will probably involve the use of non publishable internal business logic
- it will make testing new ideas faster without having to deal with the release-please repo itself.