Missing docs for photos

[jsumners]

There are not any docs for accessing photos. If we get a detailed activity through `/activities/{id}` we will get back a `photos` property that indicates:

1. The location of the primary photo

2. The number of photos uploaded to the activity

But there is not an array of photo ids. Someone on this board discovered that you can hit `/activities/{id}/photos` to get an activities photos, but there seem to be query parameters necessary to get back different photo sizes. We can guess that `?size=5000` will work, but I bet there are better ways to get back original images than guessing.

Please document the photos API.

[Jan_Mantau]

The biggest resolution I know of is 2048 and you get it with 

api/v3/activities/<activity_id>/photos?size=2048&photo_sources=1

In general you are right, this should be better documented.