Mention support for periodic exporting in README (#1811)

Co-authored-by: Kayla Reopelle <[email protected]>

Author: gjtorikian

metrics_sdk/README.md

@@ -25,14 +25,14 @@ At this time, you should be able to:
   * observable up down counters
 * Export using a pull exporter
 * Use delta aggregation temporality
+* Periodic Exporting Metric Reader
 
 We do not yet have support for:
 
 * Asynchronous instruments
 * Cumulative aggregation temporality
 * Metrics Views
 * Metrics Exemplars
-* Periodic Exporting Metric Reader
 * Push metric exporting
 
 These lists are incomplete and are intended to give a broad description of what's available.