Planet: Note in docs that download may take a long time #278

pelias download all in the planet project downloads https://data.openaddresses.io/openaddr-collected-global.zip, which is (at the time of this writing) a 12.4 GB file whose download speed seems to be throttled to 500 KB/s, which means it will take 7 or 8 hours to download. It's worth mentioning that in the docs.