Document thread safety of PluginManagers (#288)

pf4j/src/main/java/org/pf4j/AbstractPluginManager.java

@@ -37,6 +37,8 @@ import java.util.Set;
  * implementation would have to support.
  * It helps cut the noise out of the subclass that handles plugin management.
  *
+ * <p>This class is not thread-safe.
+ *
  * @author Decebal Suiu
  */
 public abstract class AbstractPluginManager implements PluginManager {

pf4j/src/main/java/org/pf4j/DefaultPluginManager.java

@@ -26,6 +26,8 @@ import java.nio.file.Paths;
 /**
  * Default implementation of the {@link PluginManager} interface.
  *
+ * <p>This class is not thread-safe.
+ *
  * @author Decebal Suiu
  */
 public class DefaultPluginManager extends AbstractPluginManager {