Add documentation for configuration file format

[synthead]

nload supports a configuration file, but the format and options are not documented.

The keys can be found here:

https://github.com/rolandriegel/nload/blob/8f92dc04fad283abdd2a4538cd4c2093d957d9da/src/app.cpp#L58-L100

...and the configuration can be set using an ini format of key = value, but I was only able to learn this from reading the source code.

[cheebusjeebus]

is nload no more developments? file for configuration is convenient. maybe nload forks? a sad day upon learn his last was 2018 and your was 2019.

@synthead what is location and what is name of configuration file you use? it need [sections]? thank you sirs