Improve reference links in README.md

Some sections of the README.md document were enhanced with full reference links to classes in action. This adjustment was done to improve clarity for users arguing the API. Now it's easier to correlate explanations with actual implementation.

README.md

@@ -35,8 +35,9 @@ For Maven, add the following to the `dependencies` block of your `pom.xml` file:
 
 ### Sending notifications
 
-The process starts with initializing the service with a subject (URL or `mailto:` prefixed e-mail address) 
-and a set of [VAPID keys](#vapid-keys), which are covered later in this document.
+The process starts with initializing the [WebPushService](src/main/kotlin/com/interaso/webpush/WebPushService.kt) with
+a subject (URL or `mailto:` prefixed e-mail address) and a set of VAPID keys, which are covered
+[later in this document](#vapid-keys).
 
 ```kotlin
 val pushService = WebPushService(
@@ -69,7 +70,8 @@ val subscriptionState = pushService.send(
 ### VAPID keys
 
 VAPID (Voluntary Application Server Identification) keys provides a method for application servers to identify
-themselves to push services. This section primarily covers the handling of these keys.
+themselves to push services. This section covers the handling of these keys with 
+[VapidKeys](src/main/kotlin/com/interaso/webpush/VapidKeys.kt) class.
 
 ```kotlin
 // Generate new keys
@@ -98,8 +100,8 @@ val privateKey = vapidKeys.pkcs8PrivateKey
 ### Using custom HTTP client
 
 You may prefer to use a different HTTP client for reasons of performance, suspendability, or familiarity.
-The example demonstrates how to use `WebPush` class to generate request headers and the encrypted body
-and how to process the response.
+The example demonstrates how to use [WebPush](src/main/kotlin/com/interaso/webpush/WebPush.kt) class to generate
+request headers and the encrypted body and how to process the response.
 
 ```kotlin
 // Setup request builder with subject and VAPID keys