doc: fix readme - make install should be run as a superuser

[GlenTiki]

Am I being pedantic? Probably. Might be worth throwing in the official repo for dummies scratching their heads for 15 minutes wondering why it won't install.

[bnoordhuis]

This is a good first step but there are a number of reasons why indiscriminately using sudo is maybe not a good idea:

1. It's not necessary (and possibly harmful) on systems where /usr/local is writable.
2. Not all systems use sudo (newbies probably don't use such systems, though.)

Maybe change it to: $ make install # or: sudo make install?

[bnoordhuis]

Oh, and please update the commit log. See CONTRIBUTING.md for guidance on what it should look like.

[silverwind]

I think it's pretty common to write [sudo] command to mark it optional.

[rvagg]

I'm not a huge fan of [sudo] but it is a bit of a convention so lgtm, @thekemkid can you squash your commits into one please?

[bnoordhuis]

Thanks, landed in ab04a43 with small changes to the commit log.