Improve Contribution.md

[OneLiteFeather]

To get more consistency in our code, we decided to use this template for deprecations which is applied to all contributions. In order to do so, you have to add this into our CONTRIBUTING.md

/**
 * This method does ...
 * @deprecated As of <product> <version>, because ... use
 *             {@link #replacementMethod()} instead.
 */
@Deprecated
public void deprecatedMethod() {
    // ...
}