layer-contents.md: Update documentation

* Raspberry Pi 3B+ support mentioned
* Add info about linux-firmware-raspbian

Signed-off-by: Andrei Gherzan <andrei@gherzan.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/docs/layer-contents.md b/docs/layer-contents.md
index e2c59a4..7a849e0 100644
--- a/docs/layer-contents.md
+++ b/docs/layer-contents.md
@@ -24,3 +24,17 @@ Note: The raspberrypi3 machines include support for Raspberry Pi 3B+.
 * rpi-test-image
   * Image based on rpi-basic-image which includes most of the packages in this
     layer and some media samples.
+
+For other uses it's recommended to base images on `core-image-minimal` or
+`core-image-base` as appropriate. The old image names (`rpi-hwup-image` and
+`rpi-basic-image`) are deprecated.
+
+## WiFi firmware blobs
+
+Be aware that the WiFi firmwares for the supported boards are not provided by
+`linux-firmware` but by a custom recipe which only packages the needed blobs
+for these boards. This is because the upstream `linux-firmware` doesn't support
+or has outdated files for the blobs we need. The recipe
+`linux-firmware-raspbian` is based on a fork of `linux-firmware` which
+includes everything we need in order to fully support the WiFi hardware. All
+machines define `MACHINE_EXTRA_RRECOMMENDS` accordingly.