Add Grain to Official Integrations in ReadMe

[adams]

See release notes and quick start guide
https://grain.com/release-note/06-18-2025
https://grainhq.notion.site/Official-Grain-MCP-Server-Quick-Start-Guide-2106689b327f81c38726d52dec5ec74e

Add Grain's official MCP server to Official Integrations list

Description

Server Details

• Server:
• Changes to:

Motivation and Context

How Has This Been Tested?

Breaking Changes

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Protocol Documentation
• My changes follows MCP security best practices
• I have updated the server's README accordingly
• I have tested this with an LLM client
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have documented all environment variables and configuration options

Additional context

[olaservo]

Thanks for your contribution to the servers list. This has been merged in this combined PR: #2264

This is a new process we're trying out, so if you see any issues feel free to re-open the PR and tag me.