Document installation via conda-forge

[mfeurer]

As explained here, Auto-sklearn can be installed via conda-forge. This should be reflected in the installation instruction.

[BastianZim]

Feedstock: https://github.com/conda-forge/auto-sklearn-feedstock

Package: https://anaconda.org/conda-forge/auto-sklearn

[satyam-jha]

i added conda installation in docs can you please check this that its ok ?
#1144

[eddiebergman]

Closed with PR #1144