docs: langchain-anthropic README updates (langchain-ai#17684)

# PR Message

- **Description:** This PR adds a README file for the Anthropic API in
the `libs/partners` folder of this repository. The README includes:
  - A brief description of the Anthropic package
  - Installation & API instructions
  - Usage examples
  
- **Issue:**
[17545](langchain-ai#17545)
  
- **Dependencies:** None

Additional notes:
This change only affects the docs package and does not introduce any new
dependencies.

---------

Co-authored-by: Bagatur <[email protected]>

libs/partners/anthropic/README.md

@@ -1 +1,34 @@
 # langchain-anthropic
+
+This package contains the LangChain integration for Anthropic's generative models.
+
+## Installation
+
+`pip install -U langchain-anthropic`
+
+## Chat Models
+
+| API Model Name     | Model Family   |
+| ------------------ | -------------- |
+| claude-instant-1.2 | Claude Instant |
+| claude-2.1         | Claude         |
+| claude-2.0         | Claude         |
+
+To use, you should have an Anthropic API key configured. Initialize the model as:
+
+```
+from langchain_anthropic import ChatAnthropicMessages
+from langchain_core.messages import AIMessage, HumanMessage
+
+model = ChatAnthropicMessages(model="claude-2.1", temperature=0, max_tokens=1024)
+```
+
+### Define the input message
+
+`message = HumanMessage(content="What is the capital of France?")`
+
+### Generate a response using the model
+
+`response = model.invoke([message])`
+
+For a more detailed walkthrough see [here](https://python.langchain.com/docs/integrations/chat/anthropic).