First half of usermod readme updates

Describe the new usermod enable process, and update sample
platformio_override.ini stubs.

usermods/LD2410_v2/readme.md

@@ -10,21 +10,15 @@ The movement and presence state are displayed in both the Info section of the we
 ## Dependencies
 - Libraries
   - `ncmreynolds/ld2410@^0.1.3`
-  - This must be added under `lib_deps` in your `platformio.ini` (or `platformio_override.ini`).
 - Data is published over MQTT - make sure you've enabled the MQTT sync interface.
 
 ## Compilation
 
-To enable, compile with `USERMOD_LD2410` defined  (e.g. in `platformio_override.ini`)
+To enable, compile with `LD2140` in `custom_usermods` (e.g. in `platformio_override.ini`)
 ```ini
 [env:usermod_USERMOD_LD2410_esp32dev]
 extends = env:esp32dev
-build_flags =
-    ${common.build_flags_esp32}
-    -D USERMOD_LD2410
-lib_deps = 
-    ${esp32.lib_deps}
-    ncmreynolds/ld2410@^0.1.3
+custom_usermods = ${env:esp32dev.custom_usermods} LD2140
 ```
 
 ### Configuration Options