docs: Add HumbleFax email-to-fax feature documentation

CRITICAL MISSING FEATURE: HumbleFax has native email-to-fax that wasn't documented!

Added comprehensive documentation for:
- Email-to-fax sending (number@humblefax.com)
- Email forwarding for received faxes
- Quick start guide with real examples
- Dual notification system (email + Faxbot webhook)
- Supported file formats and features
- Setup instructions for email forwarding

Users can now:
1. Send faxes by emailing number@humblefax.com
2. Receive faxes as email PDF attachments
3. Use both email AND API methods

This dramatically improves usability for HumbleFax users who don't
know about this convenient native feature.

Author: Faxbot Agent

docs/humblefax.md

@@ -2,6 +2,72 @@
 
 This guide summarizes the verified request format, environment variables, and end-to-end testing steps for HumbleFax.
 
+## Email-to-Fax Feature (Native HumbleFax)
+
+**⚠️ CRITICAL: HumbleFax has built-in email-to-fax functionality that Faxbot users should leverage!**
+
+This is a native HumbleFax feature that works alongside Faxbot's API integration. You get BOTH methods:
+- **Email → Fax** (HumbleFax native, super convenient)
+- **API → Fax** (Faxbot integration, programmatic control)
+
+### Quick Start: Send Your First Fax via Email
+
+1. **Get your HumbleFax account** (if you don't have one): https://humblefax.com/signup
+2. **Send an email** to: `<fax-number>@humblefax.com`
+   - Replace `<fax-number>` with digits only (no dashes, spaces, or +)
+   - **Example:** To fax +1 (303) 555-1234 → Email to `13035551234@humblefax.com`
+3. **Write your message:**
+   - Subject: Becomes the header of your cover sheet
+   - Body: Becomes the body of your cover sheet
+   - Attachments: PDF, DOC, DOCX, images, etc.
+4. **Send!** You'll receive a confirmation email when delivered
+
+### Send Faxes via Email
+Send an email to `<fax-number>@humblefax.com` where `<fax-number>` is the recipient's fax number (digits only).
+
+**Supported Formats:** PDF, DOC, DOCX, TXT, PNG, JPG, GIF, TIFF
+**Cover Sheet:** Email subject + body automatically creates a professional cover sheet
+**Confirmation:** Delivery confirmation sent to your email
+**Multiple Recipients:** CC other addresses to send to multiple fax numbers simultaneously
+
+**Real-World Example:**
+```
+To: 13035551234@humblefax.com
+Subject: Medical Records Request - Patient #12345
+Body: Please see attached lab results for John Doe (DOB: 01/15/1980)
+Attachments: lab_results.pdf
+
+→ Creates professional fax with cover sheet + attachment
+```
+
+### Receive Faxes via Email
+During HumbleFax signup, choose a local fax number in any area code. Every incoming fax is automatically sent to your email as a PDF attachment.
+
+**Dual Notification:**
+1. **Email:** Immediate PDF delivery to your inbox
+2. **Faxbot Webhook:** Updates Faxbot inbox via API (if configured)
+
+**Benefits:**
+- No busy signals (handles unlimited simultaneous faxes)
+- PDFs can be printed, saved, or forwarded instantly
+- Full history available in HumbleFax dashboard
+- Mobile-friendly: Check faxes on any device
+- Search your email to find old faxes
+
+**Setup Email Forwarding:**
+1. Log into HumbleFax dashboard: https://humblefax.com/dashboard
+2. Go to Settings → Email Notifications
+3. Enter your email address for fax delivery
+4. Save settings
+
+**Pro Tip:** Use email rules/filters to automatically organize incoming faxes by sender or subject
+
+**Learn More:** https://humblefax.com/?jump=how-it-works
+
+---
+
+## API Integration (Faxbot)
+
 Notes
 - Verified against HumbleFax quickSendFax: `multipart/form-data` with `file` field and `jsonData` object.
 - `jsonData` requires `recipients` (array of integers), `includeCoversheet`, `pageSize` (Letter|Legal|A4|B4), and typically `resolution` (Fine).