base/package-info.java: add the "deprecation exception" rule to package comment
* <li>No API dependencies on external libraries other than the core JDK</li>
* <li>Implementation dependencies on Guava are allowed for now (but should be avoided as much as possible)</li>
* </ul>
+ * <p>
+ * <b>Temporary exception:</b> In the 0.17 release, we are allowing some dependencies on other packages, e.g. to
+ * {@link org.bitcoinj.core.NetworkParameters} provided that those references are in <b>deprecated</b> methods. This will
+ * smooth migration by allowing users to, for example, replace {@code import org.bitcoinj.base.Address} with
+ * {@code import org.bitcoinj.core.Address} as first step of conversion and remove usages of the deprecated methods in
+ * a second step.
+ * <p>
* The base package will help us make bitcoinj more modular as we will use it to break circular dependencies
* between existing packages. We are also considering splitting {@code base} into a separate JAR/module in a
* future release.
