dev-manual: layers: Add notes about layer.conf

As discussed before with Richard Purdie, the code supports this but the documentation does not.
Developers in general will not notice this or focus on it because they do not mess with the
layer.conf template file, but in my opinion I think more details can help.

(From yocto-docs rev: c4b94c24ff7e1b1609f9c7f0aebd24fd04d00ae9)

Signed-off-by: Talel BELHAJSALEM <bhstalel@gmail.com>
Reviewed-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

documentation/dev-manual/layers.rst

@@ -128,6 +128,20 @@ Follow these general steps to create your layer without using tools:
       variable is a good way to indicate if your particular layer is
       current.
 
+
+   .. note::
+
+      A layer does not have to contain only recipes ``.bb`` or append files
+      ``.bbappend``. Generally, developers create layers using
+      ``bitbake-layers create-layer``.
+      See ":ref:`dev-manual/layers:creating a general layer using the \`\`bitbake-layers\`\` script`",
+      explaining how the ``layer.conf`` file is created from a template located in
+      ``meta/lib/bblayers/templates/layer.conf``.
+      In fact, none of the variables set in ``layer.conf`` are mandatory,
+      except when :term:`BBFILE_COLLECTIONS` is present. In this case
+      :term:`LAYERSERIES_COMPAT` and :term:`BBFILE_PATTERN` have to be
+      defined too.
+
 #. *Add Content:* Depending on the type of layer, add the content. If
    the layer adds support for a machine, add the machine configuration
    in a ``conf/machine/`` file within the layer. If the layer adds