Install instructions not detailed enough to work on a new Mac purchased from the store

### Before submitting an issue, please be sure to

- [x] Read [the contributing instructions](https://github.com/github/pages-gem/blob/master/docs/CONTRIBUTING.md#contributing-to-the-github-pages-gem)
- [x] Update to the latest Gem version (run `bundle update github-pages`)

### This issue affects

- [x] Building sites locally

### What did you do (e.g., steps to reproduce)

1. Go to Apple Store
2. Buy new Mac
3. Open README
4. Follow first instruction on the page, type `gem update bundler` into Terminal

### What did you expect to happen?

No error

### What happened instead?

Error

> Updating installed gems
> Updating bundler
> ERROR: While executing gem ... (Gem::FilePermissionError)

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### Notes

Please note that GitHub is used to host websites. Lots of people know how to edit HTML, they now teach this in middle school.

Not everybody knows how to set up a build environment, use git, set up Ruby, set up Ruby under Homebrew, set up Ruby under RVM under Homebrew. Not even starting on Jekyll yet.

It will be very empowering if the instructions here can help _anybody_ get started running a website locally, with full and correct instructions.