add a section about elementary particles

[certik]

With a simple explanation about each. Some info is here:

http://en.wikipedia.org/wiki/Elementary_particle

but such tables are hard to remember. I think a better approach is to list the particles that are included in the electroweak standard model*, and then particles that arise when we include quarks in it too. That should cover everything I think.

. * by listing their terms in the Lagrangian, as well as their spin (again from the Lagrangian) and so on. It should be clear where each particle comes from in the construction of the standard model