Add note to readme

[bwiernik]

It might be a good idea to add this comment to the Readme or elsewhere for any individuals who might fork or modify the portable app:

Do NOT, at any point, set AllowMultipleInstances=true inside the ZoteroPortable.ini file. This option directs Portable Zotero to be started with the -no-remote flag, which disrupts communication between Zotero and the Word/LibreOffice plugins.

[pedrom34]

Done, thanks!