add composer.json to top level for packagist submission

composer.json

@@ -0,0 +1,17 @@
+{
+  "name": "grpc/grpc",
+  "type": "library",
+  "description": "gRPC library for PHP",
+  "keywords": ["rpc"],
+  "homepage": "http://grpc.io",
+  "license": "BSD-3-Clause",
+  "require": {
+    "php": ">=5.5.0",
+    "google/auth": "dev-master"
+  },
+  "autoload": {
+    "psr-4": {
+      "Grpc\\": "src/php/lib/Grpc/"
+    }
+  }
+}