Clarify loading of minetest.conf in Readme.txt

[Fixer-007]

Clarify in doc/README.txt:

- A run-in-place build will look for the configuration file in 
        $location_of_exe/../minetest.conf and also $location_of_exe/../../minetest.conf

Addresses #5245:

[23:33:23] celeron55: a run-in-place will look for the configuration file from $location_of_exe/../minetest.conf and also $location_of_exe/../../minetest.conf
[23:33:31] celeron55: a run-in-place build*
[23:34:24] celeron55: useful for eg. visual studio builds that append another directory before bin
[23:35:58] celeron55: or for collections of different minetests that still should use the same configuration

Yay, my first pull request.

[paramat]

👍

[paramat]

0a6e53b

Note the second line was indented by the incorrect number of spaces, there was also a space after the first line, i fixed both on merge.