docs: add update schedule note to README

Author: gavinbarron

README.md

@@ -146,6 +146,10 @@ try {
 
 * [Microsoft Graph website](https://aka.ms/graph)
 
+## Update Schedule
+
+The Microsoft Graph PHP SDK is scheduled to be updated during the second and fourth week of each month
+
 ## Upgrading
 
 For detailed information on breaking changes, bug fixes and new functionality introduced during major upgrades, check out our [Upgrade Guide](UPGRADING.md)