Add setup instruction in Getting Started for Indexing (#1847)

`VectorstoreIndexCreator` [uses Chroma as the vectorstore by
default](1c22657256/langchain/indexes/vectorstore.py (L49)).
It may be helpful to add a short note for the setup.

You can see how the notebook looks here.

https://github.com/mocobeta/langchain/blob/feat/add-setup-instruction-to-index-getting-started/docs/modules/indexes/getting_started.ipynb

docs/modules/indexes/getting_started.ipynb

@@ -7,6 +7,12 @@
    "source": [
     "# Getting Started\n",
     "\n",
+    "By default, LangChain uses [Chroma](../../ecosystem/chroma.md) as the vectorstore to index and search embeddings. To walk through this tutorial, we'll first need to install `chromadb`.\n",
+    "\n",
+    "```\n",
+    "pip install chromadb\n",
+    "```\n",
+    "\n",
     "This example showcases question answering over documents.\n",
     "We have chosen this as the example for getting started because it nicely combines a lot of different elements (Text splitters, embeddings, vectorstores) and then also shows how to use them in a chain.\n",
     "\n",