Update install from source instructions?

[mdehollander]

After merging PR #44 CMakeLists.txt is not longer present and running cmake throws an error:

CMake Error: The source directory "~/src/mamba/include" does not appear to contain CMakeLists.txt.

[wolfv]

Hi, yep that's true. I need to fix the readme.

The preferred way of installing is running

pip install -e . # for dev installation
pip install . # for non dev installation

# or 

python -m pip install -e . # etc (or use setup.py directly)

that's also how the package for conda-forge is built.

[mdehollander]

Thanks! After installing gcc_linux-64 and gxx_linux-64 it compiled :)