add requirements for correct PDB files to README

[fkaiserbio]

Is your feature request related to a problem? Please describe. A problem is that users experience difficulties when dealing with custom PDB structures.

Describe the solution you'd like The README should contain a clear and descriptive list of requirements for correct PDB files, including:

• removal of CONECT entries for modified PDB files as this may result in malformed bonds being drawn in PyMol (see #79)
• retaining LINK entries from original PDB files as these are required for proper detection of metal complexes and composite ligands
• the removal of insertion codes for ligands (see #78)

Additionally, the README should give an excerpt on how such a perfect PDB file must look like.

Describe alternatives you've considered An alternative would be to drop PDB legacy format in favor of mmCIF as this format is more standardized.

Additional context n/a