add readme

Author: keyonghan

dev/README.md

@@ -0,0 +1,22 @@
+This directory contains resources that the Flutter team uses during 
+the development of engine.
+
+## Luci builder file
+`engine_try_builders.json` and `engine_prod_builders.json` contains the 
+supported luci try/prod builders for engine. It follows format:
+```json
+{
+    "builders":[
+        {
+            "name":"xxx",
+            "repo":"engine"
+        },
+        {
+            "name":"yyy",
+            "repo":"engine"
+        }
+    ]
+}
+```
+This file will be mainly used in [`flutter/cocoon`](https://github.com/flutter/cocoon)
+to trigger/update engine luci tasks.