Include a little more information on usage in the README

[ELLIOTTCABLE]

So, node-build from the homebrew-nodenv tap hasn't yet been updated to 8.6.0:

❯ System refresh-zsh* ⟰  brew update && brew upgrade node-build
Already up-to-date.
Error: node-build 2.6.13 already installed

Looking through nodenv plugins, I saw I'd already installed node-build-update-defs, so I tried that:

❯ System refresh-zsh* ⟰  [34s] nodenv update-version-defs
Checking for new nodejs versions
Checking for new nodejs release candidate versions
6.11.3 written
----8<----8<----
8.6.0 written

Unfortunately, that doesn't seem to have changed my ability to install 8.6.0:

❯ System refresh-zsh* ⟰  [12s] nodenv install 8.6.0
node-build: definition not found: 8.6.0

See all available versions with `nodenv install --list'.

If the version you need is missing, try upgrading node-build:

  brew update && brew upgrade node-build
nodenv: version `8.6.0' not installed
mkdir: /lib/node_modules/.hooks: Permission denied

I'm sure I can hunt down how to get this to work; but a glance at the README yields no information about how to use nodenv update-version-defs; I figure this is something that belongs in a README! (=

Hope this was helpful.