Readme updated

README.md

-This plugin will add 'Sent-At' header with timestamp in perf_counter format to request headers.
+This plugin will:
+
+1. Replace generic id of the request with unique one generated using uuid4.
+2. Store request start time in csv file with path given in HIVEMIND_BENCHMARKS_IDS_FILE env variable.
+
+Storage format:
+```
+request_uuid,resuest_start_time
+```
+
+This file will be read by benchmarking system to evaluate request/response time.
+