…stom Qt distribution. Without this option, linuxdeployqt is not able to look at the Qt plugins directory. (#463)

* Add -qtlibinfix support in order to precise the libinfix set for a custom Qt distribution. Without this option, linuxdeployqt is not able to look at the Qt plugins directory.

* Add dedicated qtlibinfix section in documentation.

* Missing lib prefix for the -qtlibinfix example in documentation.

* Update README.md

Co-authored-by: Tristan Cladet <tristan.cladet@siemens.com>
Co-authored-by: probonopd <probonopd@users.noreply.github.com>