Document the need to `include Primer::ViewHelper` in order to use the helpers

[kjg]

The Octicon docs mention possible use of the primer_octicon helper and the PrimerOcticon cop corrects all usage of the octicon helper to be primer_octicon instead, but the docs don't make it clear how to get the primer_octicon helper to work.

It appears that one must either have include Primer::ViewHelper in their ApplicationHelper or helper Primer::ViewHelper in their ApplicationController (or any other more specific controller). I'd suggest the Getting Started page is updated to include this info as part of the Installation

[lesliecdubs]

👋🏻 Hi @kjg, thanks for reporting this! We would welcome a PR to the docs; I believe this is the file to edit. Let us know if we can help!

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