Add Lerna convertion warning

.github/pull_request_template.md

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+⚠️ ATTENTION ☢️ Uppy is [transitioning into a Lerna repo](https://github.com/transloadit/uppy/pull/906) this/next week, please wait with new PRs to avoid conflicts. Discussion in [#862](https://github.com/transloadit/uppy/issues/862)