Add guideline for Anaconda or Miniconda users

[HannoSpreeuw]

When installing amuse, Anaconda and Miniconda users can get into trouble because of mixing of conda-forge and non-conda-forge packages.
In my case I ran into more undefined references to '__isoc23_strtol' when trying to install PeTaR: ./setup install amuse-petar.
This error indicates a versioning problem with the strtol function, which comes from string.h.

The source of this problem seems to come from the way I created my Miniconda environment for amuse with Python 3.13, i.e. the default way:

conda create --name amuse_p313 python=3.13

This problem can be circumvented by forcing conda-forge:

conda create --name amuse_p313 python=3.13 --channel conda-forge --override-channels

while still using Miniconda.