Small update for Pymakr plugin description for Atom

[Batilan]

The current description for installing the Pymakr plugin for Atom at the website can be improved a little, it seems to be geared to the Mac version of Atom as it only mentions "Atom > Preferences > Install". Note that "Install" is found on different places depending on the platform:
Linux: Edit > Preferences
MacOS: Atom > Preferences
Windows: File > Settings

[bucknall]

Hi @Batilan,

Noted! Will make this change to the docs!

Thanks,

Alex