add readme for openapi

openapi/Readme.md

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+
+## Grafana
+
+### query (return by DataFrame) (for IoTDB plugin)
+
+query condition:
+series paths, start time, end time, group by time interval, limit
+
+### query (return by Json) (for SimplyJson plugin)
+
+query condition:
+series paths, start time, end time, group by time interval, limit
+
+### getChildren  (return next level of a path)
+
+query condition: the path of a parent (support wildcard '*', )
+
+return: a set of children name
+
+root -> ng-bj, ng-sh, sg-sz, sg-gz
+
+root.ng-bj -> 100KW, 1000KW, 10MW
+
+root.ng-sh -> 100KW, 1000KW, 10MW
+
+### concretePaths
+
+root.*.100KW ->
+root.ng-bj.100KW root.ng-sh.100KW
+
+
+