In pipelines: mention transformers caching to avoid duplicate computations

@mikolajr https://scikit-learn.org/stable/modules/compose.html#caching-transformers-avoid-repeated-computation

changed milestone to %Workshop 3rd edition (2020-2021)

assigned to @mikolajr

created merge request !28 (merged) to address this issue

mentioned in merge request !28 (merged)

closed with merge request !28 (merged)

mentioned in commit 786149db