docs: Add global install instructions, change phrasing

[RichardLitt]

I have added the way to globally install node-sass from the CLI,
which isn't present in the README otherwise. I have also removed a
sentence which says that node-sass is simplistic. In both cases,
it is assumed that someone using the program knows how to use npm
to install command line packages, and that the user has some idea
about the level of complexity usual to an npm package. Both of
these assumptions are assumptions - my wording makes the package
more friendly to beginners by not assuming they know this.

tl;dr: just a small change to help new users.

[xzyfer]

my wording makes the package more friendly to beginners by not assuming they know this

I whole heartedly support this sentiment. However we do not recommend, or support install node-sass globally.

[RichardLitt]

How would the CLI client work, otherwise?

[xzyfer]

The primary usecase for the CLI is in npm scripts. Alternatively you can use something like npx.

However global installs of node-sass have been notoriously troublesome and are not something we'd like to encourage.

[RichardLitt]

How about the language I just pushed?