[k32w0] Add OTA encryption info in contact sensor README

Contact sensor uses some gn args to enable OTA encryption.
This means that an upcoming OTA image must be encrypted
with the same key provided by chip_with_ota_key.

Signed-off-by: marius-alex-tache <marius.tache@nxp.com>

Author: marius-alex-tache

examples/contact-sensor-app/nxp/k32w/k32w0/README.md

@@ -643,6 +643,10 @@ application. Please make sure that the application is written at address 0x4000:
 
 ![FLASH_LOCATION](../../../../platform/nxp/k32w/k32w0/doc/images/flash_location.JPG)
 
+Please also note that, by default, the device expects the OTA image to be
+encrypted with the same key specified by `chip_with_ota_key`. See `args.gni` for
+the default gn configuration.
+
 ### OTA Testing
 
 The OTA topology used for OTA testing is illustrated in the figure below.
@@ -730,9 +734,13 @@ Please see more in the
 Here is an example that generates an OTA image with application update TLV:
 
 ```
-./scripts/tools/nxp/ota/ota_image_tool.py create -v 0xDEAD -p 0xBEEF -vn 42021 -vs "1.0" -da sha256 --app-input-file chip-k32w0x-contact-example.bin chip-k32w0x-contact-example.ota
+./scripts/tools/nxp/ota/ota_image_tool.py create -v 0xDEAD -p 0xBEEF -vn 2 -vs "2.0" -da sha256 --enc_enable --input_ota_key "1234567890ABCDEFA1B2C3D4E5F6F1B4" --app-input-file chip-k32w0x-contact-example.bin chip-k32w0x-contact-example.ota
 ```
 
+Please note the two options `--enc_enable` and `--input_ota_key`, which are
+mandatory when `chip_with_ota_encryption=1`. The value of `--input_ota_key` must
+match the value of `chip_with_ota_key`.
+
 A note regarding OTA image header version (`-vn` option). An application binary
 has its own software version, given by
 `CHIP_DEVICE_CONFIG_DEVICE_SOFTWARE_VERSION` (`42020` by default), which can be