Readme - Add hint of how to upgrade the Wicket or Spring Boot Version without waiting for a new release

[MarcGiffing]

[MarcGiffing]

https://github.com/MarcGiffing/wicket-spring-boot/issues/149#issuecomment-459780242

[mweirauch]

If it helps, here's what I am using when not using any or another parent:

    <dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-dependencies</artifactId>
                <version>${spring-boot.version}</version>
                <type>pom</type>
                <scope>import</scope>
            </dependency>
            <dependency>
                <groupId>org.apache.wicket</groupId>
                <artifactId>wicket-parent</artifactId>
                <version>${wicket.version}</version>
                <type>pom</type>
                <scope>import</scope>
            </dependency>
        </dependencies>
    </dependencyManagement>