enable 3D Ball sample import button in package manager for unity sdk (#5665)

* enable 3D Ball sample import button in package manager for unity sdk

* add Samples folder

* add meta file

* fix git ignore config

Author: xcao65

com.unity.ml-agents/.gitignore

@@ -10,6 +10,7 @@ npm-debug.log
 build.sh.meta
 build.bat.meta
 .idea/
+!Samples/*/*.unitypackage
 
 /[Ll]ibrary/
 /Logs/
@@ -21,8 +22,6 @@ build.bat.meta
 /Assets/Plugins*
 /Assets/Demonstrations*
 /csharp_timers.json
-/Samples/
-/Samples.meta
 
 # Visual Studio 2015 cache directory
 /.vs/

com.unity.ml-agents/Samples.meta

@@ -0,0 +1 @@
+{"displayName":"3D Ball","description":"The 3D Ball sample is a simple environment that is a great for jumping into Ml-Agents to see how things work."}

com.unity.ml-agents/Samples/3DBall.meta

@@ -8,5 +8,12 @@
     "com.unity.barracuda": "2.3.1-preview",
     "com.unity.modules.imageconversion": "1.0.0",
     "com.unity.modules.jsonserialize": "1.0.0"
-  }
+  },
+  "samples": [
+    {
+      "displayName": "3D Ball",
+      "description": "The 3D Ball sample is a simple environment that is a great for jumping into Ml-Agents to see how things work.",
+      "path": "Samples/3DBall"
+    }
+  ]
 }