Add notes about new symbol used in Unofficial-search-plugins.mediawiki

[flyxyz123]

Tokyo Toshokan used new symbol ❌ to show it breaks qbt plugin system as shown in here. However, it is not documented in the "Read First" section.

[flyxyz123]

Another maybe better way is to change symbol ❌ to symbol ✖ in Tokyo Toshokan's section. I noticed this because the new symbol ❌ breaks my old script that auto download good unofficial search plugins and update every month. This use of new symbol caused my script to download bad tokyotoshokan.py which breaks my search plugin system for three months, and I was only able to fix it after spending hours until I saw that Tokyo Toshokan may break search plugin system, so I removed tokyotoshokan.py and it worked.

But I do understand that red ❌ can get more attention than grey ✖. And the tokyotoshokan.py issue is very serious, so the use of ❌ is a good fit here. Thus my pull request is to add documentation about ❌ instead of replace ❌ with ✖.

[thalieht]

Another maybe better way is to change symbol ❌ to symbol ✖ in Tokyo Toshokan's section.

IMO either this or change all ✖ to ❌. Having 2 X is pointless.