fix: add summary, on-the-go instructions and #build requirement

Signed-off-by: Jasper Lutz Severino <jasperlutzseverino@gmail.com>

README.md

@@ -10,6 +10,27 @@ automatically on Discord.
 
 ## Summary
 
+- [What is this?](#what-is-this-)
+- [Summary](#summary)
+- [Installation](#installation)
+  * [Available spins](#available-spins)
+  * [Versions](#versions)
+- [Features](#features)
+- [Usage](#usage)
+  * [Instantiating the library](#instantiating-the-library)
+    + [Available default database technologies](#available-default-database-technologies)
+  * [Creating the book](#creating-the-book)
+  * [Adding pages](#adding-pages)
+  * [Customizing buttons](#customizing-buttons)
+  * [Adding component rows](#adding-component-rows)
+  * [Building the book](#building-the-book)
+  * [Registering the books](#registering-the-books)
+  * [Sending the books](#sending-the-books)
+  * [Sending on-the-go books](#sending-on-the-go-books)
+  * [Listening to the button event](#listening-to-the-button-event)
+  * [Handling errors](#handling-errors)
+- [Credits](#credits)
+
 ## Installation
 
 Add the following to your `pom.xml` file:
@@ -215,6 +236,22 @@ public class Example extends JDABook {
 }
 ```
 
+### Building the book
+
+After all of that is done, we call the `#build()` method to stitch everything together.
+
+```java
+public class Example extends JDABook {
+    public Example() {
+        super("example");
+
+        // {...}
+
+        build();
+    }
+}
+```
+
 ### Registering the books
 
 To register the books, you call the `#registerBooks` method.
@@ -228,7 +265,6 @@ public class Main {
     }
 }
 ```
-
 ### Sending the books
 
 That was easy, right? Let's send the books!
@@ -238,9 +274,30 @@ be `JDAReceivable`.
 
 ```java
 JDAReceivable jdaReceivable=new JDAReceivable(event.getChannel());
-        jdaReceivable.receive(DiscordBooks.getBook("example"));
+jdaReceivable.receive(DiscordBooks.getBook("example"));
 ```
 
+### Sending on-the-go books
+
+Books don't necessarily need to be registered to be sent, you can create and send books on the spot.
+
+```java
+public class Main {
+    public static void main(String[] args) {
+        JDAReceivable jdaReceivable = new JDAReceivable(event.getChannel());
+
+        jdaReceivable.receive(new Book()
+                .addPages(
+                        new SendableImpl("This is a temporary book!"),
+                        new SendableImpl("With variable content, right " + event.getAuthor().getAsTag() + "?")
+                ).build());
+    }
+}
+```
+
+This book will be stored in the temporary database. Read [the first usage chapter](#instantiating-the-library) for more
+information.
+
 ### Listening to the button event
 
 The library comes with a pre-made listener for the buttons, you can use it by adding it to your respective library