Link Issue events in /activity/events/

The [Activity Event documentation](http://developer.github.com/v3/activity/events) says:

    All Events have the same response format

This is true for all Event endpoint **except one**:

    GET /repos/:owner/:repo/issues/events

In fact, this endpoint returns the following fields: `id` (integer), `actor`, `commit_id`, `created_at`, `event`, `issue`, `url`, while all the other endpoints return the following fields: `id` (string), `type`, `actor`, `repo`, `payl

To avoid confusion, I suggest to *add a link* to the Issue Events page
that has an example of the response format returned for Issue Events
Merge pull request #334 from claudiob/patch-1

Link Issue events in /activity/events/