docs: add README with usage instructions

Author: fulleni

README.md

@@ -0,0 +1,66 @@
+# ht_email_sendgrid
+
+![coverage: percentage](https://img.shields.io/badge/coverage-100-green)
+[![style: very good analysis](https://img.shields.io/badge/style-very_good_analysis-B22C89.svg)](https://pub.dev/packages/very_good_analysis)
+[![License: PolyForm Free Trial](https://img.shields.io/badge/License-PolyForm%20Free%20Trial-blue)](https://polyformproject.org/licenses/free-trial/1.0.0)
+
+A concrete implementation of the `HtEmailClient` interface that uses the SendGrid API to send transactional emails.
+
+## Description
+
+This package provides `HtEmailSendGrid`, a class that connects to the SendGrid v3 API to send emails. It is designed to be used with the `ht_email_repository` and `ht_http_client` packages, leveraging the standardized `HtHttpClient` for making HTTP requests and handling errors.
+
+## Getting Started
+
+Add the dependency to your `pubspec.yaml`:
+
+```yaml
+dependencies:
+  ht_email_sendgrid:
+    git:
+      url: https://github.com/headlines-toolkit/ht-email-sendgrid.git
+      # ref: <specific_tag_or_commit>
+```
+
+## Usage
+
+To use this client, you must first configure an `HtHttpClient` instance with the SendGrid API base URL and an `AuthInterceptor` that provides your SendGrid API key.
+
+```dart
+import 'package:dio/dio.dart';
+import 'package:ht_email_client/ht_email_client.dart';
+import 'package:ht_email_sendgrid/ht_email_sendgrid.dart';
+import 'package:ht_http_client/ht_http_client.dart';
+import 'package:logging/logging.dart';
+
+void main() async {
+  // 1. Configure the HtHttpClient for SendGrid
+  final sendGridHttpClient = HtHttpClient(
+    baseUrl: 'https://api.sendgrid.com/v3',
+    tokenProvider: () async => 'YOUR_SENDGRID_API_KEY', // Provide your key here
+    isWeb: false, // Use false for server-side applications
+  );
+
+  // 2. Instantiate the SendGrid email client
+  final HtEmailClient emailClient = HtEmailSendGrid(
+    httpClient: sendGridHttpClient,
+    log: Logger('SendGridClient'),
+  );
+
+  // 3. Use the client to send an email
+  try {
+    await emailClient.sendTransactionalEmail(
+      recipientEmail: 'recipient@example.com',
+      templateId: 'd-your-template-id',
+      templateData: {'name': 'World'},
+    );
+    print('Email sent successfully!');
+  } on HtHttpException catch (e) {
+    print('Failed to send email: $e');
+  }
+}
+```
+
+## License
+
+This package is licensed under the [PolyForm Free Trial](LICENSE). Please review the terms before use.