Documentation update

[scottmcarthur-heb]

Added documentation to explain how to use Github access tokens with this wagon. Github API V3 has changed the way that authentication works with access tokens.

Instruction summary:

Github Auth with Access Token

When using an access token, the Github V3 API expects that the token be in the Authorization header as a bearer token. Add server entries into your settings.xml file like this:

<settings>
  ...
  <servers>
    <server>
    <id>your-repo-id</id>
    <configuration>
      <httpHeaders>
        <property>
          <name>Authorization</name>
          <value>Bearer ${github.accesstoken}</value>
        </property>
      </httpHeaders>
    </configuration>
    </server>
  ...
  </servers>
  ...
</settings>