Document no pip install on Windows

[daikitag]

I'm using Windows laptop, and I can install msprime on my computer by using conda, but I cannot install msprime by using pip, as the following message shows up:

ERROR: Could not build wheels for msprime, which is required to install pyproject.toml-based projects

Would it be possible for you to fix the issue, such that msprime can be pip installed on a windows laptop?