advertise new conda-forge installation option

[untzag]

PyDAQmx is newly available through conda on conda-forge. This PR updates the docs so that people are aware of this option.

conda-forge works by having a "feedstock" repository for each package. A bot comes around and builds the package from this repository. It's not a lot of work to maintain---here's the feedstock for PyDAQmx

https://github.com/conda-forge/pydaqmx-feedstock

Currently the only maintainers of this feedstock are myself and my colleague @ksunden, neither of us are PyDAQmx developers. I'd be happy to add others or relinquish my maintainership of this feedstock to anyone else just let me know.