Update README for v0.4.0 (#422)

As we prepare to release v0.4.0, update the README to reference the new
version.

Author: findleyr

README.md

@@ -1,13 +1,13 @@
 <!-- Autogenerated by weave; DO NOT EDIT -->
-# MCP Go SDK v0.3.0
+# MCP Go SDK v0.4.0
 
 [![Open in GitHub Codespaces](https://github.com/codespaces/badge.svg)](https://codespaces.new/modelcontextprotocol/go-sdk)
 
 ***BREAKING CHANGES***
 
 This version contains breaking changes.
 See the [release notes](
-https://github.com/modelcontextprotocol/go-sdk/releases/tag/v0.3.0) for details.
+https://github.com/modelcontextprotocol/go-sdk/releases/tag/v0.4.0) for details.
 
 [![PkgGoDev](https://pkg.go.dev/badge/github.com/modelcontextprotocol/go-sdk)](https://pkg.go.dev/github.com/modelcontextprotocol/go-sdk)
 

internal/readme/README.src.md

@@ -1,12 +1,12 @@
-# MCP Go SDK v0.3.0
+# MCP Go SDK v0.4.0
 
 [![Open in GitHub Codespaces](https://github.com/codespaces/badge.svg)](https://codespaces.new/modelcontextprotocol/go-sdk)
 
 ***BREAKING CHANGES***
 
 This version contains breaking changes.
 See the [release notes](
-https://github.com/modelcontextprotocol/go-sdk/releases/tag/v0.3.0) for details.
+https://github.com/modelcontextprotocol/go-sdk/releases/tag/v0.4.0) for details.
 
 [![PkgGoDev](https://pkg.go.dev/badge/github.com/modelcontextprotocol/go-sdk)](https://pkg.go.dev/github.com/modelcontextprotocol/go-sdk)