Add MINIO_REGION to server help message (#4558)

* Add e.g for setting MINIO_REGION env variable
* Add MINIO_REGION to region table

cmd/server-main.go

@@ -58,6 +58,9 @@ ENVIRONMENT VARIABLES:
   BROWSER:
      MINIO_BROWSER: To disable web browser access, set this value to "off".
 
+  REGION:
+     MINIO_REGION: To set custom region. By default it is "us-east-1".
+
 EXAMPLES:
   1. Start minio server on "/home/shared" directory.
       $ {{.HelpName}} /home/shared

docs/config/README.md

@@ -48,7 +48,14 @@ minio server ~/Photos
 #### Region
 |Field|Type|Description|
 |:---|:---|:---|
-|``region``| _string_ | `region` describes the physical location of the server. By default it is set to `us-east-1`, which is same as AWS3's default region. If you are unsure leave it to default.|
+|``region``| _string_ | `region` describes the physical location of the server. By default it is set to `us-east-1`, which is same as AWS S3's default region. You may override this field with `MINIO_REGION` environment variable. If you are unsure leave it unset.|
+
+Example:
+
+```sh
+export MINIO_REGION="my_region"
+minio server ~/Photos
+```
 
 #### Browser
 |Field|Type|Description|