Update README.rst

[okaragoz]

• [x] Couldn't find $ sudo apt-get install pyqt5-dev-tool and $ sudo apt-get install python-sphinx packages for installation instructions.
• [x] Fixed a minor typo mistake $ sudo apt-get install pyqt5-dev-tools.
• [x] Updated the command used to install the Sphinx library $ sudo apt-get install python3-sphinx.
• [x] Added command line codes for different distros (RHEL/Centos/Fedora) to install required packages.