Install yarn with conda

Author: jtpio

docs/source/examples/Widget Custom.ipynb

@@ -53,7 +53,7 @@
     "To create the environment, execute the following command:\n",
     "\n",
     "```bash\n",
-    "conda create -n ipyemail -c conda-forge jupyterlab cookiecutter nodejs python\n",
+    "conda create -n ipyemail -c conda-forge jupyterlab cookiecutter nodejs yarn python\n",
     "```\n",
     "\n",
     "Then activate the environment with:\n",