add documentation that makes it clear what the different tags contain

[SvenDowideit]

The readme does not confirm to the user that the jre versions are only the jre (for example)

listing the image size along with the tags would help with it, as the sizes would be very obviously more inline with whats in them.

[yosifkit]

Image size is a bit complex as the docs are not generated on the build server and we would probably need to download all the images to get their sizes. It will definitely be a good thing to have. I think @tianon has some ideas to prevent downloading the whole images.

[tianon]

Finally had a chance to play with my bad ideas: docker-library/docs#86