clarify the usage of S3 credentials

README.md

@@ -348,6 +348,17 @@ Planetary Computer uses the same signed URL pattern described above. Here is an
   }
 }
 ```
+### Simple S3 authentication
+To use simple S3 authentication one has to set some environmental variables with S3 credentials:
+
+```AWS_SECRET_ACCESS_KEY```
+```AWS_ACCESS_KEY_ID```
+
+or to specify:
+
+[user profile](https://docs.aws.amazon.com/cli/v1/userguide/cli-configure-files.html#cli-configure-files-format) with a proper reference to ```AWS_PROFILE``` in the file ```AWS_CONFIG_FILE```. For more info please visit [GDAL vsis3 documentation](https://gdal.org/en/latest/user/virtual_file_systems.html#vsis3-aws-s3-files). 
+
+
 ## Contributing
 
 All contributions are subject to the