Add LUCI try builders json config file (flutter#2993)

* add json config

* mv to .ci

Author: keyonghan

.ci/dev/README.md

@@ -0,0 +1,19 @@
+This directory contains resources that the Flutter team uses during 
+the development of plugins.
+
+## Luci builder file
+`try_builders.json` contains the supported luci try builders 
+for plugins. It follows format:
+```json
+{
+    "builders":[
+        {
+            "name":"yyy",
+            "repo":"plugins",
+            "enabled":true
+        }
+    ]
+}
+```
+This file will be mainly used in [`flutter/cocoon`](https://github.com/flutter/cocoon) 
+to trigger/update pre-submit luci tasks.

.ci/dev/try_builders.json

@@ -0,0 +1,9 @@
+{
+   "builders":[
+      {
+         "name":"Windows Plugins",
+         "repo":"plugins",
+         "enabled":true
+      }
+   ]
+}