Add hint to use/update/branch a new manifest at that point

[s-hemer]

doc-bsp-yocto/source/yocto/kirkstone.rst

Lines 1445 to 1458 in ae86016

	.. code-block:: console
	host:~$ cd sources
	host:~$ git clone git://git.yoctoproject.org/meta-security
	host:~$ cd meta-security
	host:~$ git branch -r
	All available remote branches will show up. Usually there should be 'fido',
	'jethro', 'krogoth', 'master', ...
	.. code-block:: console
	host:~$ git checkout kirkstone

It may make sense to highlight at this point, that this adding of additonal (own) layers should be represented/nailed-down in a manifest.

Also this section has a lot of similarities to the following on adding an own layer, maybe squash:

doc-bsp-yocto/source/yocto/kirkstone.rst

Lines 1535 to 1558 in ae86016

	Default options are fine for now. Move the layer to the source directory
	.. code-block:: console
	host:~$ mv meta-racer ../sources/
	Create a *Git* repository in this layer to track your changes
	.. code-block:: console
	host:~$ cd ../sources/meta-racer
	host:~$ git init && git add . && git commit -s
	Now you can add the layer directly to your build/conf/bblayers.conf
	.. code-block:: kconfig
	BBLAYERS += "${BSPDIR}/sources/meta-racer"
	or with a script provided by *Yocto*
	.. code-block:: console
	host:~$ bitbake-layers add-layer meta-racer