Add documentation entry for AWS_STS_REGIONAL_ENDPOINTS

Signed-off-by: Elijah Zupancic <e.zupancic@f5.com>

Author: dekobon

docs/getting_started.md

@@ -51,6 +51,10 @@ There are few optional environment variables that can be used.
 
 * `HOSTNAME` - (optional) The value will be used for Role Session Name. The default value is nginx-s3-gateway.
 * `STS_ENDPOINT` - (optional) Overrides the STS endpoint to be used in applicable setups. This is not required when running on EKS. See the EKS portion of the guide below for more details.
+* `AWS_STS_REGIONAL_ENDPOINTS` - (optional) Allows for a regional STS endpoint to be
+  selected. When the regional model is selected then the STS endpoint generated will
+  be coded to the current AWS region. This environment variable will be ignored if
+  `STS_ENDPOINT` is set. Valid options are: `global` (default) or `regional`.
 
 
 ### Configuring Directory Listing