Add information about open development process

The development process is entirely transparent and it may be helpful to highlight this aspect of KDE software in the BE applications.

For instance, meaningful commit messages linked to issues affect maintainability and autonomy of use. Moreover, all users are encouraged and able to express their needs, wishes, and aims for the future of Okular development and participate in the planning of future milestones and therewith the overall evolution of Okular.

Some relevant links:

• Overview: https://invent.kde.org/graphics/okular
• Bugs: https://bugs.kde.org
• Issues: https://invent.kde.org/graphics/okular/-/issues
• Mailing list: https://mail.kde.org/mailman/listinfo/okular-devel
• Code of Conduct: https://kde.org/code-of-conduct/