Document manual pharmr installation

[rikardn]

Document manual pharmr installation on webpage or pharmr README:

1. Choose if you want to use conda or pip/venv (base python packages) (what follows is for the second option)
2. If pip/venv install Python. I would recommend winpython for Windows. They have some nice minimal installers.
3. Setup a virtual environment. For example using python -m venv myvenvdirectory
4. In the command line activate the environment: Scripts\activate.bat
5. In the same command line window: pip install pharmpy-core
6. Change the setting in rstudio (under options/Python) to point to the newly created venv.
7. You can use reticulate::py_discover_config() to check that everything is setup correctly