docs(install): add save-peer flag (#7616)

Author: drew4237

docs/lib/content/commands/npm-install.md

@@ -134,6 +134,8 @@ into a tarball (b).
 
     * `-D, --save-dev`: Package will appear in your `devDependencies`.
 
+    * `--save-peer`: Package will appear in your `peerDependencies`.
+
     * `-O, --save-optional`: Package will appear in your
       `optionalDependencies`.