Add more explicit information on enabling parallelization in quanteda >v4.0.0

[stefan-mueller]

Parallelisation on Linux, macOS, and Windows now works, but users need to have TBB installed.

Currently, the README lists these installation instructions under Compile from source, but I think we should also put these instructions in a separate section on Enabling parallelisation. We could describe this upfron in README and maybe in a short stand-alone tutorial under "Examples" at quanteda.io.

Any views or suggestions, @kbenoit? Happy to take care of this.

[kbenoit]

Yes very good idea, thanks!