docs: readme for langchain-mistralai (#14917)

- **Description:** Add README doc for MistralAI partner package.
  - **Tag maintainer:** @baskaryan

libs/partners/mistralai/README.md

@@ -1 +1,41 @@
 # langchain-mistralai
+
+This package contains the LangChain integrations for [MistralAI](https://docs.mistral.ai) through their [mistralai](https://pypi.org/project/mistralai/) SDK.
+
+## Installation
+
+```bash
+pip install -U langchain-mistralai
+```
+
+## Chat Models
+
+This package contains the `ChatMistralAI` class, which is the recommended way to interface with MistralAI models.
+
+To use, install the requirements, and configure your environment.
+
+```bash
+export MISTRAL_API_KEY=your-api-key
+```
+
+Then initialize
+
+```python
+from langchain_core.messages import HumanMessage
+from langchain_mistralai.chat_models import ChatMistralAI
+
+chat = ChatMistralAI(model="mistral-small")
+messages = [HumanMessage(content="say a brief hello")]
+chat.invoke(messages)
+```
+
+`ChatMistralAI` also supports async and streaming functionality:
+
+```python
+# For async...
+await chat.ainvoke(messages)
+
+# For streaming...
+for chunk in chat.stream(messages):
+    print(chunk.content, end="", flush=True)
+```