fix: show in console that UpmGitExtensions v2 is not available in Unity2018 or 2019

close #117

Author: mob-sakai

Editor/Legacy.meta

@@ -0,0 +1,17 @@
+{
+    "name": "Coffee.UpmGitExtension.LegacyWarning",
+    "references": [],
+    "includePlatforms": [
+        "Editor"
+    ],
+    "excludePlatforms": [],
+    "allowUnsafeCode": false,
+    "overrideReferences": false,
+    "precompiledReferences": [],
+    "autoReferenced": false,
+    "defineConstraints": [
+        "!UNITY_2020_1_OR_NEWER"
+    ],
+    "versionDefines": [],
+    "noEngineReferences": false
+}

Editor/Legacy/Coffee.UpmGitExtension.LegacyWarning.asmdef

@@ -0,0 +1,17 @@
+using UnityEngine;
+using UnityEditor;
+
+namespace Coffee.UpmGitExtension
+{
+    public class LegacyWarning
+    {
+        [InitializeOnLoadMethod]
+        private static void Log()
+        {
+#if !UNITY_2020_1_OR_NEWER
+            Debug.LogError("'UpmGitExtensions package (v2)' does not support Unity 2018 or 2019. Please install 'UpmGitExtensions package (v1)' instead.\n" +
+                "For details, see https://github.com/mob-sakai/ParticleEffectForUGUI#installation-for-unity-2018-or-2019");
+#endif
+        }
+    }
+}