Add note to README about two-way sync

[dvanoni]

From a discussion on a PR:

@pmmendoza, that's a good question! Unfortunately, it's not possible for the plugin to be notified of any updates within Notion.

The question of syncing updates from Notion back into Zotero was also asked by someone in the Zotero forums. It would be neat, but it's unfortunately beyond the scope of the plugin. Getting updates from Notion into Zotero would require setting up a hosted service that subscribes to webhooks from Notion and then uses the Zotero API to update items in Zotero. Notion has yet to release official webhook support, but there are some third-party tools that can be used for this. In theory this is technically possible, but it would be a separate project from the plugin and something I wouldn't be able to take on anytime soon.

Originally posted by @dvanoni in #35 (comment)

[pmmendoza]

Understood - didn't know about the complexity of this ✓;
I can still update things easily in Zotero - that's def. sufficient for now 👏