update install doc

docs/installation.md

@@ -3,6 +3,20 @@
 ## Gradle
 ```kotlin
 // build.gradle.kts
+
+buildscript {
+    repositories {
+        maven { url = uri("https://jitpack.io") }
+    }
+    dependencies {
+        classpath("com.github.flecomte:postgres-json:+")
+    }
+}
+
+repositories {
+    maven { url = uri("https://jitpack.io") }
+}
+
 dependencies {
     implementation("com.github.flecomte:postgres-json:+")
 }