README: clarify line about .olean files

[waldyrious]

Just a small tweak to make the line less cryptic for beginners.

Although, to be honest I still don't fully understand what this sentence is saying — how does one make sure the .olean files are generated in the _target folder, and what exactly will be extremely slow otherwise?

[bryangingechen]

I usually run leanproject up to update the project to depend on the latest mathlib master, or simply leanproject get-mathlib-cache if I don't care what version of mathlib I'm generating the docs for.

See https://leanprover-community.github.io/leanproject.html#getting-mathlib-oleans

[bryangingechen]

what exactly will be extremely slow otherwise?

If the .olean files are not present, generating the docs will take a long time since Lean will have to compile all of mathlib from scratch.

[jcommelin]

Which easily takes 2 hours.