Add rate limiting documentation to API docs

[tompscanlan]

Problem

The API has rate limiting but it's not documented anywhere visible to API consumers.

Solution

Add a section to the README or create a dedicated docs file explaining:

• Which endpoints are rate limited
• What the limits are
• How rate limit headers work
• What to do when rate limited

Files to create/modify

• docs/RATE-LIMITING.md (new file)
• Update README.md to link to it

Content to include

# Rate Limiting

## Default Limits
- General API: X requests per minute
- Authentication: X requests per minute  
- File uploads: X requests per minute

## Response Headers
- `X-RateLimit-Limit`: Maximum requests allowed
- `X-RateLimit-Remaining`: Requests remaining
- `X-RateLimit-Reset`: Unix timestamp when limit resets

## When Rate Limited
You'll receive a 429 status code. Wait until the reset time before retrying.

Acceptance Criteria

• Rate limits are documented
• Examples show how to handle 429 responses
• README links to new documentation

Difficulty: Easy - documentation task, good for learning the codebase