Create documentation

docs/usage/raw-request.md

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+# Raw request
+You can execute query directly from the code like this:
+(*see [Init connection](./init-connection.md) before*)
+
+```kotlin
+import fr.postgresjson.connexion.Connection
+
+val connection: Connection = TODO()
+
+val result: QueryResult = connection.exec(
+    "SELECT id FROM inventor WHERE name = :name",
+    mapOf("name" to "Nikola Tesla")
+)
+val id: String = result.rows[0].getString(0)
+```
+
+And if you must map the query result with an entity, you can do it like this:
+```kotlin
+import java.util.UUID
+import fr.postgresjson.entity.Serializable
+import fr.postgresjson.connexion.Connection
+
+val connection: Connection = TODO()
+
+data class Inventor(
+    val id: UUID = UUID.randomUUID(),
+    val name: String
+): Serializable
+
+val result: Inventor? = connection.selectOne(
+    """
+    SELECT json_build_object(
+        'id', '9e65de49-712e-47ce-8bf2-dfffae53a82e', 
+        'name', :name
+    )
+    """,
+    mapOf("name" to "Nikola Tesla")
+)
+
+val inventor = connection.selectOne<Inventor>("SELECT * FROM mytable WHERE id = :id")
+
+val inventors: List<Inventor> = connection.select("SELECT * FROM mytable WHERE status = 'done'")
+```
+
+
+See [ConnectionTest.kt](/src/test/kotlin/fr/postgresjson/ConnectionTest.kt) for more examples.