Hello! Thank you for the amazing library you've created!
While following the tutorial at [the link(`Using an example
selector`)](https://langchain.readthedocs.io/en/latest/modules/prompts/examples/few_shot_examples.html#using-an-example-selector),
I noticed that passing Chroma as an argument to from_examples results in
a type hint error.
Error message(mypy):
```
Argument 3 to "from_examples" of "SemanticSimilarityExampleSelector" has incompatible type "Type[Chroma]"; expected "VectorStore" [arg-type]mypy(error)
```
This pull request fixes the type hint and allows the VectorStore class
to be specified as an argument.
Also updated docs, and noticed an issue with the add_texts method on
VectorStores that I had missed before -- the metadatas arg should be
required to match the classmethod which initializes the VectorStores
(the add_example methods break otherwise in the ExampleSelectors)
Jan