Add multi-library to readme

README.md

@@ -245,6 +245,31 @@ kept. Proxies count as owned copies.
 > **Update card data**, and **Import card data…** too on a phone. Dismissing it
 > with **Not now** lasts until a later version needs something more.
 
+## Libraries
+
+Every device starts with one library, **Main**. To keep others alongside it —
+a scratch library to try things in, a cube, a trade binder — click the
+library name at the left of the top bar and choose **New library…**. The same
+menu switches between them; **Info → Libraries** lists them all, and is where
+you rename or delete them (on a phone, it's the only place).
+
+Each library has its own cards and decks; the selected deck and the
+"location" for new cards are remembered per library too. Card data is shared.
+
+A new library can **sync** to your other devices or stay on this one. Synced
+libraries appear on every signed-in device by themselves; one kept on this
+device only never leaves it, which makes it the safe place to experiment.
+The **Sync** checkbox in **Info → Libraries** changes your mind either way.
+
+Deleting a synced library asks whether to **remove it from this device** (it
+stays on the server and on your other devices, and this one stops downloading
+it) or **delete it everywhere**. Other devices keep their copy of a library
+deleted everywhere, marked as deleted from the server, until you remove it or
+put it back. **Main** can be renamed but not deleted.
+
+Libraries' files live in the app-data dir: **Main** where the one library
+always lived, the others under `libraries/`.
+
 ## Sync
 
 Your library always lives on the device you're using, so the app works the