Implement block executor

.luacheckrc

@@ -5,6 +5,6 @@ std = {
         'getmetatable', 'setmetatable', 'rawget', 'print', 'shard_status',
         'loadstring', 'arg',
     },
-    globals = {'package'}
+    globals = {'package', '_G'}
 }
 redefined = False

README.md

@@ -124,6 +124,12 @@ local compiled_query = graphql_lib.compile(query)
 local result = compiled_query:execute(variables)
 ```
 
+### Shard
+
+One need to call `require('graphql.storage').init()` on each storage server to
+use graphql with shard and BFS executor. Alternatively one can use
+`graphql.new({..., use_bfs_executor = 'never'})` on a frontend server.
+
 ### Multi-head connections
 A parent object is matching against a multi-head connection variants in the
 order of the variants. The parent object should match with a determinant of
@@ -404,6 +410,14 @@ TEST_RUN_TESTS=common/mutation make test
 * For building apidoc (additionally to 'for use'):
   * ldoc.
 
+## Hacking
+
+Enable debug log:
+
+```sh
+export TARANTOOL_GRAPHQL_DEBUG=1
+```
+
 ## License
 
 Consider LICENSE file for details. In brief: