Add LUCI try builders json config file

.ci/dev/README.md

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+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

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+{
+   "builders":[
+      {
+         "name":"Windows Plugins",
+         "repo":"plugins",
+         "enabled":true
+      }
+   ]
+}