Installation Page Clarification

This issue is in relation to https://github.com/chocolatey/home/issues/18

We should add some notes on the installation page about _why_ retrying the installation of chocolatey proceeds to fail if you don't run the installer as an admin at first.

The issue stems from the necessary folders being _created_ during the first run, which also is the idempotence check we use identify that chocolatey is already installed.

Docs is a good start here, but we may want to be a _little_ more intentional about how we check for an existing in the installation scripts overall.