Add documentation: Clarify Authentication Methods and API Key Availability

[quantstruct-dev-sarthak]

Authentication

This document explains how to authenticate with the API. Proper authentication is required to access any of the API endpoints.

Introduction

Currently, the API uses a combination of X-API-Key and X-API-Secret for authentication. These headers must be included in every request you make to the API.

Important: API keys are currently not self-service. Please contact the founders to request your API key and secret. We are working towards providing a self-service option in the future. Check back here for updates.

Quickstart

To make a successful API call, you'll need to include the X-API-Key and X-API-Secret headers in your request. Here's an example using curl:

curl -X POST \
  'https://your-api-endpoint/v1/messages/individual/{accountId}/send' \
  -H 'Content-Type: application/json' \
  -H 'X-API-Key: YOUR_API_KEY' \
  -H 'X-API-Secret: YOUR_API_SECRET' \
  -d '{
    "content": "Hello, world!",
    "attachment_uri": null,
    "from": "