Portable configuration files support added. Updated README.MD

[shakir-timur]

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• Searched the bugtracker for similar pull requests
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• Bug fix
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Description of your pull request and other information

3 years ago I created a feature request that would allow to store config files in the same folder with executable (#18324). After some googling I decided to do it myself. :-)
Additionally I updated the appropriate README.md section, which in my opinion does not describe thoroughly all the possible ways of storing the configuration file.