Added information for RHEL 9 compability

[rizajur]

updated README.md

preview : https://github.com/rizajur/Flatseal/blob/master/README.md

for dirty workaround regarding broken font implementation of OS default interface font (Gnome 40.1) would be cool if we could change the font family to something dejavu related in the future depending on the availability of the required packages in the different operating systems or if embedding is best option

[tchx84]

Apologies for the late review, would you mind moving this to the documentation file instead? Under the "tips and tricks" section, or add a "Troubleshoot" section.