docs(plugins): add listing for new plugin (#1686)

Author: amanda-mitchell

docs/extending/plugins-list.md

@@ -108,3 +108,8 @@
 - [semantic-release-npm-deprecate-old-versions](https://github.com/ghusse/semantic-release-npm-deprecate-old-versions)
   - `verifyConditions`: Validates configuration.
   - `publish`: Deprecates old versions, based on the declaration of supported versions in the config. 
+- [amanda-mitchell/semantic-release-npm-multiple](https://github.com/amanda-mitchell/semantic-release-npm-multiple)
+  - **Note**: this is a thin wrapper around the built-in npm plugin that can target multiple registries
+  - `verifyConditions`: Verify the presence and the validity of the npm authentication and release configuration for multiple registries
+  - `prepare`: Update the package.json version and create the npm package tarball
+  - `publish`: Publish the package on the npm registry for multiple registries