Update the initial README

Signed-off-by: Ignas Anikevicius (gns_ank) <anikevicius@gmail.com>
PTA data model mostly done.

This is code to the PTA model as described by Ellis et al. There are
several functions left to do and some clarification is needed as well.

The current issues are:

    - How to put in pulsar angular coordinates efficiently and elegantly
    - Finish coding some of the functions
    - Add a noise function (random gaussian white noise)

Signed-off-by: Ignas Anikevicius (gns_ank) <anikevicius@gmail.com>