Add README.md files for multiple @kit packages

Added README.md files for several @kit packages including Supabase, UI, Emails, Billing, and Mailers. These documents explain the purpose and usage of each package, facilitating better understanding for developers interacting with them.

packages/mailers/README.md

@@ -0,0 +1,33 @@
+# Mailers - @kit/mailers
+
+This package is responsible for sending emails using a unified interface across the app.
+
+The default mailer uses the `nodemailer` package to send emails. You can create custom mailers by extending the `Mailer` class.
+
+Make sure the app installs the `@kit/mailers` package before using it.
+
+```json
+{
+    "name": "my-app",
+    "dependencies": {
+        "@kit/mailers": "*"
+    }
+}
+```
+
+## Usage
+
+### Send an email
+
+```javascript
+import { Mailer } from '@kit/mailers';
+
+const mailer = new Mailer();
+
+mailer.send({
+    to: '',
+    from: '',
+    subject: 'Hello',
+    text: 'Hello, World!'
+});
+```