add docs for authenication

README.md

@@ -1,6 +1,6 @@
 # github-email
 
-Retrieve a GitHub user's email even if it's not public. 
+Retrieve a GitHub user's email even if it's not public.
 Pulls info from Github user, NPM, activity commits, owned repo commit activity.
 
 ![image](https://cloud.githubusercontent.com/assets/39191/7485758/6992dc62-f34f-11e4-9af0-3d0f292f6139.png)
@@ -13,6 +13,17 @@ npm install --global github-email
 
 ### use
 ```sh
-github-email user 
+github-email ghusername
 ```
 
+# Authenication Token
+
+If the `GH_EMAIL_TOKEN` environment variable is not set, the script will ask
+the user to generate a personal access token [for authentication](https://developer.github.com/v3/auth/#basic-authentication). To do this:
+
+1. Visit https://github.com/settings/tokens/new?description=github-email
+1. Keep the checkboxes all unchecked
+1. Click __Generate Token__.
+1. Copy the token.
+1. Run this in your shell `export GH_EMAIL_TOKEN=<token>`
+1. Also add that line to your shell configuration (e.g. .bashrc)