Add sudo

[snyff]

Since building the binary should be done using a low-privileges account, administrator will need to sudo (or any other way to escalate privileges) to install brotli.

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[AppVeyorBot]

✅ Build brotli 1.0.0#1424 completed (commit 7501b3b590 by @snyff)

[eustas]

Thanks for your contribution.

Unfortunately, it looks wrong to advice users to use 'sudo'. Some might get suspicious.
Usually, non-priviliged users already know that they have to set prefix to their user dir. Privileged users have "sudo" reflex for "make install" =)