Update README: npm package does not support --embedded flag (#2022)

Author: jmooring

README.md

@@ -419,7 +419,11 @@ an API for users to invoke Sass and define custom functions and importers.
 * `sass --embedded --version` prints `versionResponse` with `id = 0` in JSON and
   exits.
 
-No other command-line flags are supported with `--embedded`.
+The `--embedded` command-line flag is not available when you install Dart Sass
+as an [npm package]. No other command-line flags are supported with
+`--embedded`.
+
+[npm package]: #from-npm
 
 ## Behavioral Differences from Ruby Sass