Update the readme on pipe example

[lcarasik]

The pipe example: https://github.com/Nek5000/NekExamples/tree/master/pipe states there is three different cases (i.e. helix, cyclic, and stenosis) but it isn't specifically stated what each one is for. It would be good to revise the case for a basic 3-D straight pipe flow in addition to the current examples.