Readme.md: build and usage guide inconsistent

[thomas725]

Your build guide tells to build in a separate subfolder "build", but the usage guide assumes that the built backscrub binary is in the same folder as the models directory

To make the usage guide compatible with the build guide, you either have to add the path to the model:

./backscrub -d -d -c /dev/video0 -v /dev/video10 -m ../models/selfiesegmentation_mlkit-256x256-2021_01_19-v1215.f16.tflite -b ../images/background_bauhaus.png

or you start it from outside the build directory:

build/backscrub -d -d -c /dev/video0 -v /dev/video10 -b images/background_bauhaus.png

[BenBE]

It's more complicated than that, actually. ;-) There is some magic on IIRC experimental (soon to lang on main) that goes without the paths completely and looks for those directories itself. So you would need relative paths only if things were outside the default directories of the tool.

[phlash]

It's probably a hangover from the Makefile build that dropped the binary in $cwd and which I think we ought to retire as it's not feature compatible with the CMake build (no XNNPACK support).

That said - it always helps to have working cut'n paste docs 😄

[phlash]

Related to #127, when the README.md gets updated there we should fix this (if required)