By leave of court, upon reasonable notice and upon such terms as are just, or by written consent of the adverse party, a party may serve and file a supplemental pleading setting forth transactions or occurrences or events which have happened since the date of the pleading sought to be supplemented. Leave may be granted even though the original pleading is defective in its statement of a claim for relief or defense. No responsive pleading to the supplemental pleading is required unless the court, upon its own motion or the motion of a party, so orders, specifying the time therefor.