Add rust as a dependency

[gglanzani]

On my Mac, I had to install rust as well to install through pip.

[jongwook]

Thanks, rust install is required only when a pre-built tokenizers binary is not available, so I moved the paragraph to below.

[flockonus]

@jongwook I didn't have rust on my machine, so I followed the install link on README but just that setup wasn't sufficient to complete installation, still getting:

Collecting tokenizers!=0.11.3,<0.13,>=0.11.1
  Using cached tokenizers-0.12.1.tar.gz (220 kB)
  Installing build dependencies ... done
  Getting requirements to build wheel ... error
  error: subprocess-exited-with-error
  
  × Getting requirements to build wheel did not run successfully.
  │ exit code: 1
  ╰─> [17 lines of output]

...