Long_description in setup.py

[ZdenekM]

Currently, the long_description field is generated by concatenating README and CHANGELOG while building packages:

https://github.com/robofit/arcor2/blob/6dab12458e1081407989f61b34da71b9840be12d/pants-plugins/setup_kwargs_plugin/register.py#L69

However, it makes the setup.py very long (in some cases) and it would be better to put there something like:

long_description=read(README.md)

...in order to read the file content in runtime.