Extension authors guide should mention the need for quarkus-extension-processor

[mkouba]

A user should add something like to the pom.xml of the deployment module:

<build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.7.0</version> <!-- 3.5+ is required -->
                <configuration>
                    <annotationProcessorPaths>
                        <path>
                            <groupId>io.quarkus</groupId>
                            <artifactId>quarkus-extension-processor</artifactId>
                        </path>
                    </annotationProcessorPaths>
                </configuration>
            </plugin>
        </plugins>
</build>