Document shipping just .nim files
I am interested in trying all ways of shipping that nimporter can do, one of the options is not documented along with the others,
thats the option of just shipping the `.nim` files, I made a PIP package that install Nim and other that can install Nimpy,
so would be nice to document on the readme how users can package the `.nim` files.
For new users it might not be so obvious.

You can make your package depend on it `install_requires = ["choosenim_install"]`, heres an example on Windows:

![windows-compile](https://user-images.githubusercontent.com/1189414/107665391-26e8f080-6c6c-11eb-9fa1-742265c1d57b.png)

Commands in order to copy & paste:
- `pip install choosenim_install`
- `pip install nimble_install --install-option="--nimble=nimpy"`

Feel free to use the screenshot if is useful for Docs.
:)
