Add warning to docs regarding NPM publish bug

doc/user/packages/npm_registry/index.md

@@ -181,6 +181,9 @@ Add a corresponding section to your `.npmrc` file:
 
 ## Uploading packages
 
+DANGER: **Warning:**
+Due to a [bug in NPM](https://github.com/npm/cli/issues/1994), version `7.x` and later do not respect the `publishConfig` entry in the `package.json` file. To publish, you must use an earlier version of NPM, or temporarily set your `.npmrc` scope to `@foo:registry=https://gitlab.example.com/api/v4/projects/<project_id>/packages/npm`. 
+
 Before you can upload a package, you need to specify the registry
 for NPM. To do this, add the following section to the bottom of `package.json`: