Add more detailed instructions for setting up on GHE

We have a document that we've used internally for setting up on GHE, and I think it would make sense to include here.

Here's the raw text, that would need some editing to fit this situation.

```
# Setting Up GitHub for Developers Class on GHE

The following steps are designed to help admins set up the necessary accounts, repositories, and permissions to allow learners to complete GitHub's training classes on your organization's GitHub Enterprise instance.

## Step 1: Create Necessary Accounts

- Create an individual account named `githubteacher`. This is the account the trainer will use.
- Create an organization called `githubschool`. Githubteacher must be an admin in this organization.

## Step 2: Clone Template Repositories

Clone the following repositories from github.com:

https://github.com/githubtraining/caption-this
https://github.com/githubtraining/github-games
https://github.com/githubtraining/polygons

## Step 3: Push Template Repositories to GHE

1. Sign in as `githubteacher`
2. In the `githubschool` organization, create three new repositories with the same names as the ones you cloned above (`caption-this`, `github-games`, and `polygons`). **Do not choose the option to Initialize the repo with a README**
3. Copy the clone URLs provided for each repository to your clipboard.
4. From the command line, cd into each repository you cloned in step 2.
5. Type `git remote set-url origin GHE-URL`
- Note: in the `polygons` reposoitory, you will need to checkout to each of the branches before you push.
6. Type `git push --mirror origin`.
7. Please ensure the pushed repositories contain the appropriate branches (most have more than 1).

## Step 4: Set up GitHub pages

Several of our class repositories are published as [GitHub pages](https://pages.github.com/). Please [enable GitHub Pages on your GHE instance](https://help.github.com/enterprise/2.12/admin/guides/installation/configuring-github-enterprise-pages/).

## Step 5: Invite Participant collaboratores to `caption-this`

We will collaborate on the `caption-this` repository in class. Please gather the GHE usernames of each class participant and invite them as collaborators to the `caption-this` repository. You may find it easier to create a team in the new org, and invite individuals to that team. This will allow you to grant access to newly created repos in the future.

>Note: this is only necessary for the `caption-this` repository, as we will fork the other repositories.
```

If anyone would like to tackle this before I can get to it, please feel free!