Make the user aware of the parameters generation

[schomatis]

Both in terms of computer resources and user's time. A note should be added in the Getting Started guide. And if possible, provide instructions for the user to download them instead, through paramfetch (if that tool is meant for the normal user).

This can be very frustrating for a new user excited to try Filecoin for the very first time and stumbling upon a very expensive computation for which it wasn't notified of nor it understands its motivation.

See #1998 as an example.

[anorth]

We don't intend users to need to generate the parameters, this is a (hopefully brief) interim. See #1975.

[schomatis]

I see, but this interim exists as of today (even if it will hopefully be solved soon), so maybe it's still worth adding a note to the user testing Filecoin now.

[rosalinekarr]

Thanks to PR #1975, we now download params instead of generating them, so I'm closing this.