docs: add note about BCrypt hashes

Related: https://github.com/socketio/socket.io-admin-ui/issues/9

README.md

@@ -92,6 +92,27 @@ instrument(io, {
 });
 ```
 
+WARNING! Please note that the `bcrypt` package does not currently support hashes starting with the `$2y$` prefix, which is used by some BCrypt implementations (for example https://bcrypt-generator.com/ or https://www.bcrypt.fr/). You can check the validity of the hash with:
+
+```
+$ node
+> require("bcrypt").compareSync("<the password>", "<the hash>")
+true
+```
+
+You can generate a valid hash with:
+
+```
+$ node
+> require("bcrypt").hashSync("changeit", 10)
+'$2b$10$LQUE...'
+```
+
+See also:
+
+- https://github.com/kelektiv/node.bcrypt.js/issues/849
+- https://stackoverflow.com/a/36225192/5138796
+
 #### `namespaceName`
 
 Default value: `/admin`