Add documentation for paginated and multi level request

docs/usage/multi-level.md

@@ -0,0 +1,93 @@
+Multi Level Queries
+===================
+
+## Define schema, query and kotlin object
+1. Schema
+```postgresql
+create table parent (
+    id uuid primary key,
+    name text not null
+);
+
+create table child (
+    id uuid primary key,
+    name text not null,
+    parent_id uuid not null references parent
+)
+```
+2. Insert some data for tests
+```postgresql
+insert into parent (id, name) VALUES ('379e0687-9e4a-4781-b0e9-d94a62e4261f', 'Bernard');
+insert into child (id, name, parent_id) VALUES (uuid_generate_v4(), 'Noé', '379e0687-9e4a-4781-b0e9-d94a62e4261f');
+insert into child (id, name, parent_id) VALUES (uuid_generate_v4(), 'John', '379e0687-9e4a-4781-b0e9-d94a62e4261f');
+```
+3. Define Model
+```kotlin
+import java.util.UUID
+
+class Parent(val id: UUID, val name: String, val children: List<Child>)
+class Child(val id: UUID, val name: String)
+```
+
+4. Define request function
+```postgresql
+-- resource/sql/functions/find_parent_by_id.sql
+create or replace function find_parent_by_id(in _id uuid, out resource json) language plpgsql as
+$$
+begin
+    select to_json(t) into resource
+    from (
+        select 
+            p.*,
+            json_agg(to_jsonb(c) - 'parent_id') as children   
+        from parent p
+        join child c on c.parent_id = p.id
+        where p.id = _id
+        group by p.id
+    ) t;
+end;
+$$;
+```
+
+## Execute the function
+
+You just to use `Requester` and set the sql function name, then pass arguments.
+
+If you need to return more than one entry, use `.select()` instead of `.selecteOne()`
+
+See the [Paginated example](./paginated.md)
+```kotlin
+import fr.postgresjson.connexion.Requester
+
+val requester: Requester = TODO()
+val result: Parent = requester
+    .getFunction("find_parent_by_id")
+    .selectOne("id" to "379e0687-9e4a-4781-b0e9-d94a62e4261f")
+```
+
+The requester create dynamically this request
+```postgresql
+select * from find_parent_by_id(_id => '379e0687-9e4a-4781-b0e9-d94a62e4261f');
+```
+*Watch the underscore as prefix is added if necessary.
+The requester known the parameters because it parses all SQL functions and reads the names of the parameters from them.*
+
+
+And the SQL return is a JSON like follow:
+```json
+{
+  "id": "379e0687-9e4a-4781-b0e9-d94a62e4261f",
+  "name": "Bernard",
+  "child": [
+    {
+      "id": "c2d0ec81-7cac-4689-8086-2644a3b309b5",
+      "name": "Noé"
+    },
+    {
+      "id": "255d911a-0cbc-4156-bf8c-0204e89494d9",
+      "name": "John"
+    }
+  ]
+}
+```
+But the requester deserialize the result automatically into a Kotlin object with their children objects. **And do that in only one request**.