Update README.md with Writer (#474)

Author: samjulien

README.md

@@ -37,7 +37,7 @@ Strands Agents is a simple yet powerful SDK that takes a model-driven approach t
 ## Feature Overview
 
 - **Lightweight & Flexible**: Simple agent loop that just works and is fully customizable
-- **Model Agnostic**: Support for Amazon Bedrock, Anthropic, LiteLLM, Llama, Ollama, OpenAI, and custom providers
+- **Model Agnostic**: Support for Amazon Bedrock, Anthropic, LiteLLM, Llama, Ollama, OpenAI, Writer, and custom providers
 - **Advanced Capabilities**: Multi-agent systems, autonomous agents, and streaming support
 - **Built-in MCP**: Native support for Model Context Protocol (MCP) servers, enabling access to thousands of pre-built tools
 
@@ -151,6 +151,7 @@ Built-in providers:
  - [LlamaAPI](https://strandsagents.com/latest/user-guide/concepts/model-providers/llamaapi/)
  - [Ollama](https://strandsagents.com/latest/user-guide/concepts/model-providers/ollama/)
  - [OpenAI](https://strandsagents.com/latest/user-guide/concepts/model-providers/openai/)
+ - [Writer](https://strandsagents.com/latest/documentation/docs/user-guide/concepts/model-providers/writer/)
 
 Custom providers can be implemented using [Custom Providers](https://strandsagents.com/latest/user-guide/concepts/model-providers/custom_model_provider/)