added todo list and updated readme

To configure, you can set in the file ~/.panorc the following options.

        shader_dir=<path to directory containing the shader files>

        image_file=<path to the desired input file>

        max_fov=<starting value of max_fov, in degrees>

Lines in the file must start with option names, and no spaces should go

between option names, equal signs and values. If no shader path is set, the

default is ./shaders/. If no input file is set, or the file is missing, a

file chooser will be shown at startup.

file chooser will be shown at startup. The default value of max_fov is 60

degrees.

Higher priority:

-Study singular points of the transformations and fix the image in those

cases.

Lower priority:

-Discretize grid, add a parameter grid_discr=5x5, 10x10, 15x15, ...

-Add a parameter for the grid resolution.

-Compute a limit for the discretization (298 degrees?).

-Change squares by triangles on the top and on the bottom of the sphere.