update readme to reflect that LAST_USED_AGE is optional

Author: sheenamt

README.md

@@ -14,7 +14,7 @@ Sleuth runs periodically, normally once a day in the middle of business hours. S
 
 - Inspect each Access Key based on:
   - set creation age threshold (default 90 days)
-  - set last accessed age threshold (default 30 days)
+  - set last accessed age threshold (optional, set to creation age threshold as default)
 - If Access Key is approaching threshold will ping user with a reminder to cycle key
 - If key age is at or over threshold will disable Access Key along with a final notice
 
@@ -106,7 +106,7 @@ The behavior can be configured by environment variables.
 | ENABLE_AUTO_EXPIRE | Must be set to `true` for key disable action |
 | EXPIRATION_AGE | Age of key creation (in days) to disable a AWS key |
 | WARNING_AGE | Age of key creation (in days) to send notifications, must be lower than EXPIRATION_AGE |
-| LAST_USED_AGE | Age of last key usage (in days) to send notifications, must be lower than or equal to EXPIRATION_AGE |
+| LAST_USED_AGE | OPTIONAL, defaults to EXPIRATION_AGE, Age of last key usage (in days) to send notifications, must be lower than or equal to EXPIRATION_AGE |
 | MSG_TITLE | Title of the notification message |
 | MSG_TEXT | Instructions on key rotation |
 | SLACK_URL | Incoming webhook to send notifications to |