docs: Add multiple unique keys FAQ recipe

docs/sites/faq.rst

@@ -3,3 +3,28 @@ FAQ
 
 Whenever you find out something that you were surprised by or needed some non-trivial
 thinking, please add it here.
+
+How do I define additional unique keys in a ``dy.Schema``?
+----------------------------------------------------------
+
+By default, ``dataframely`` only supports defining a single non-nullable (composite) primary key in ``dy.Schema``.
+However, in some scenarios it may be useful to define additional unique keys (which support nullable fields and/or which are additionally unique).
+
+Consider the following example, which demonstrates two rules: one for validating that a field is entirely unique, and another for validating that a field, when provided, is unique.
+
+::
+
+    class UserSchema(dy.Schema):
+        user_id = dy.UInt64(primary_key=True, nullable=False)
+        username = dy.String(nullable=False)
+        email = dy.String(nullable=True)  # Must be unique, or null.
+
+        @dy.rule(group_by=["username"])
+        def unique_username() -> pl.Expr:
+            """Username, a non-nullable field, must be total unique."""
+            return pl.len() == 1
+
+        @dy.rule()
+        def unique_email_or_null() -> pl.Expr:
+            """Email must be unique, if provided."""
+            return pl.col("email").is_null() | pl.col("email").is_unique()