[si ckip] Copy git warning from Paper (#54)

settings.gradle.kts

@@ -11,6 +11,23 @@ plugins {
     id("org.gradle.toolchains.foojay-resolver-convention") version "0.4.0"
 }
 
+if (!file(".git").exists()) {
+    val errorText = """
+        
+        =====================[ ERROR ]=====================
+         The Folia project directory is not a properly cloned Git repository.
+         
+         In order to build Folia from source you must clone
+         the Folia repository using Git, not download a code
+         zip from GitHub.
+         
+         See https://github.com/PaperMC/Paper/blob/master/CONTRIBUTING.md
+         for further information on building and modifying Paper and Forks.
+        ===================================================
+    """.trimIndent()
+    error(errorText)
+}
+
 rootProject.name = "folia"
 
 for (name in listOf("Folia-API", "Folia-Server")) {