doc: document testing Puter with email

doc/testing_with_email.md

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+# Testing with Email
+
+Testing anything involving email is really simple using [mailhog](https://github.com/mailhog/MailHog)
+
+### Step 1: Configure email service
+
+In your `config.json` for Puter (`volatile/config/config.json` usually, `/var/puter/config.json` in containers),
+add this entry to the `"services`" map:
+
+```javascript
+    "services": {
+        
+        // ... there are probably other service configs
+        
+        "email": {
+            "host": "localhost",
+            "port": 1025
+        }
+    }
+```
+
+### Step 2: Install and run mailhog
+
+Follow the instructions on [MailHog](https://github.com/mailhog/MailHog)'s
+repository, or install through your distro's package manager.
+
+Run the command: `mailhog`.
+
+You should now have an inbox at [http://127.0.0.1:8025](http://127.0.0.1:8025).
+
+Every email that Puter sends will show up on this page.