Improve SST setup instructions

Updated instructions for enabling SST to include setting the SST_ROOT environment variable and using the --recurse-submodules flag.

Author: hokeun

docs/reference/security.mdx

@@ -108,15 +108,14 @@ With `comm-type: SST`, both the initial connection setup and subsequent federate
 
 ### Enabling SST
 
-To use SST, first clone the iotauth repository and initialize its submodules:
+To use SST, first clone the iotauth repository and initialize its submodules, and optionally, set the environment variable `SST_ROOT` to the path of the cloned repository:
 
 ```bash
-git clone https://github.com/iotauth/iotauth.git
-cd iotauth
-git submodule update --init
+git clone --recurse-submodules https://github.com/iotauth/iotauth.git
+export SST_ROOT="$(pwd)/iotauth"
 ```
 
-Then set the `comm-type` to `SST` and provide the path to your `iotauth` installation:
+Then set the `comm-type` to `SST` and provide the path to your local `iotauth` repository:
 
 <ShowIfs>
 <ShowIf c>
@@ -262,4 +261,4 @@ If you would like to go back to non-AUTH mode, you would have to remove all cont
 Pluggable communication security (TLS and SST) is currently only supported for the **C** and **Python** targets. Support for other target languages is planned for the future.
 :::
 
-</ShowOnly>
+</ShowOnly>