docs: Add README for TRMNL API specification

- Document OpenAPI portal link and API documentation resources
- Explain authentication methods (device-level vs user-level)
- Reference implementation files in the codebase

project-resources/trmnl-api/README.md

@@ -0,0 +1,25 @@
+# TRMNL API Specification
+
+This directory contains the OpenAPI specification for the TRMNL API.
+
+## Resources
+
+- **OpenAPI Portal:** https://trmnl.com/api-docs/index.html
+- **API Documentation:** https://docs.usetrmnl.com/go
+- **Private API Docs:** https://docs.usetrmnl.com/go/private-api/introduction
+
+## Files
+
+- `trmnl-openapi.yaml` - OpenAPI 3.0.1 specification exported from the TRMNL API portal
+
+## Authentication
+
+The TRMNL API uses two authentication methods:
+
+1. **Device-level authentication** - Uses `Access-Token` header with device API key
+   - Implemented in `TrmnlApiService.kt`
+   - Used for device operations (display, setup, etc.)
+
+2. **User-level authentication** - Uses `Authorization: Bearer` header with account API key
+   - Implemented in `TrmnlUserApiService.kt`
+   - Used for account operations (device management, plugin settings, etc.)