Added the API definition for checking the index existence

rest-api-spec/api/indices.exists.indices.json

@@ -1,17 +1,18 @@
 {
   "indices.exists.indices": {
-    "documentation": "",
+    "documentation": "http://www.elasticsearch.org/guide/reference/api/admin-indices-indices-exists/",
     "methods": ["HEAD"],
     "url": {
       "path": "/{index}",
       "paths": ["/{index}"],
       "parts": {
         "index": {
+          "type" : "list",
+          "required" : true,
+          "description" : "A comma-separated list of indices to check"
         }
       },
       "params": {
-        "index": {
-        }
       }
     },
     "body": null