Add tip to the Readme for using `citar-open-entry-in-zotero` with a manual install of Zotero

[jpggithub]

I suggest to add the following advice in the Opening reference entries section of the Readme:

For using citar-open-entry-in-zotero with a manual install of Zotero, you could need the following command in the shell (as a simple user):

xdg-mime default zotero.desktop x-scheme-handler/zotero

(found on https://forums.zotero.org/discussion/comment/359181/#Comment_359181)

I have searched for approximately 2 hours before finding the problem on my PC 😅