doc: explain how to install from source

[thiswillbeyourgithub]

Signed-off-by: thiswillbeyourgithub 26625900+thiswillbeyourgithub@users.noreply.github.com

Hi,

I took the liberty of insisting a bit more on how to install from source as the pypi version is outdated (2017!)

See #103

[nbehrnd]

@thiswillbeyourgithub By September 8th, to some degree, the incentive is gone because the record on PyPI was incremented to 1.0.5, and a pip install works fine (example in Linux Debian 14/forky). Not in charge to decide about the PR here (the blessed repository), your note to use pip install git+https://github.com/mcs07/PubChemPy (instead of calling git clone, cd, and pip install . I was used to) however is noteworthy to me because it is more compact. Thank you for this one!