Installation instructions should mention additional dependencies

[Martin Modrák]

As of this writing, the installation instructions when using conda simply mention to run:

conda install -c conda-forge muq

There are two things I would have liked to know when running my installation:

1. that cmake is needed (at least to run the C++ examples)

   1. Or maybe just say that for running the examples, see the “Build from source” instructions?

2. whether there are requirements on the Python version (as I was creating a new conda virtualenv and wanted to know which Python to use)

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[Martin Modrák]

Documentation has been improved.