README file : Date range for solar system objects

[tostt]

I think it would be useful to mention in the README file that there is a date cutoff when requiring the paths of solar system objects.

The cutoff dates are mentionned in the ephemerisCompute repository but I think a reminder in the star-charter project would be informative, maybe providing the actual JD range that is OK to use.

I tried plotting paths outside of that range and got unexpected results. The program could also log a warning in this case so users know about it.