Release 2.5.1 - Readme updates & path fixes

The biggest change in this Mini-Release is the updated README. English is now up-to-date with all current options and features (and also got a new look to be less overwhelming), while Japanese and Chinese still lag behind. If you speak those languages, help translating would be much appreciated.

Also, finally a long-standing issue with Gradio security restrictions was fixed / worked around, which previously prevented the extension from working with a custom --data-dir specified in the webui launch args:
You will need to add the --gradio-allowed-path argument to your webui start script to make it work, else the helper will not throw the error anymore, but the server will still return 403 Unauthorized for any extension's JavaScript file or tags file the webui tries to load.

A word of caution if you share the webui on your local network or even as a public server:
The allowed path will be publicly exposed. A simple JavaScript fetch("file=path/to/file") can access any file contents in that directory and its subdirectories, as long as the filename is known. You need to be careful what you allow in there.
I suggest only allowing the install dir of tag autocomplete itself for base functionality, or if you use wildcard extensions and want to use the wildcard completion feature, the whole extensions dir at most.