Create documentation

docs/usage/init-connection.md

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+# Init connection
+
+Before execute any query you must instantiate the connection.
+```kotlin
+import fr.postgresjson.connexion.Connection
+
+val connection = Connection(
+    host = "localhost",
+    port = 5432,
+    database = "mydb",
+    username = "john",
+    password = "azerty"
+)
+```