Add authentication section for gdrive NPX

[po-shu]

Description

gdrive --> README.md --> Usage with Desktop App --> NPX, Authentication section are not documented.

Server Details

• Server: gdrive
• Changes to: README.md only

Motivation and Context

NPX users may become confused.

How Has This Been Tested?

• Verified README formatting
• Confirmed command and descriptions are accurate
• Tested with Claude Desktop

Breaking Changes

No breaking changes - documentation update only.

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

FYI, due to unreleased bugfixes, it won't work with NPX until the package is published.
#402 (comment)