Explain how to set PATH

[RMeli]

Given that some installations create binary files in the downloaded folder, I think it would be beneficial to explain to the students how to add such folders to their PATH on both Linux/macOS and Windows.

Related to #138

[IAlibay]

I wonder if it would make sense at this point to separate out the instructions so that we can go into more details about everything?

[RMeli]

Yes, that would make sense IMO.

[RMeli]

@IAlibay Can you change the tag to something more general? I think this might be useful regardless of docking.

[IAlibay]

Done :)