Better document the FetchContent-usage in the example

README.md

@@ -33,8 +33,10 @@ cmake_minimum_required(VERSION 3.14)
 
 include(FetchContent)
 
+# Update the commit to point to whatever libsodium-cmake-commit you want to target.
 FetchContent_Declare(Sodium
     GIT_REPOSITORY https://github.com/robinlinden/libsodium-cmake.git
+    GIT_TAG 99f14233eab1d4f7f49c2af4ec836f2e701c445e # HEAD as of 2022-05-28
 )
 set(SODIUM_DISABLE_TESTS ON)
 FetchContent_MakeAvailable(Sodium)