dev-manual: Add info on build env initialization

Going through the docs we a new trainee, we noticed that the build
initialization is only mentioned in the quick build
(https://docs.yoctoproject.org/brief-yoctoprojectqs/index.html) but not
in the in-depth getting started documentation.

While this is repeatedly mentioned later on, e.g. in the "building" section,
you might easily end up somewhere where this has not been mentioned yet.

E.g. in our case this was:

https://docs.yoctoproject.org/dev-manual/layers.html# (which comes right
after the "setup" chapter)
->
https://docs.yoctoproject.org/dev-manual/layers.html#creating-a-general-layer-using-the-bitbake-layers-script
(which is linked right at the top)

To avoid any confusion I think it best to mention this required step
directly in the "start" document.

(From yocto-docs rev: d7a36b7b8e770ae45b991698d588f9522ba59b4e)

Signed-off-by: Jasper Orschulko <jasper@fancydomain.eu>
Reviewed-by: Michael Opdenacker <michael@opdenacker.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/dev-manual/start.rst

@@ -853,3 +853,14 @@ similar to checking out by branch name except you use tag names.
    ``checkout`` command are a snapshot of the "&DISTRO_NAME_NO_CAP;"
    development branch at the point where Yocto Project &DISTRO; was
    released.
+
+Initializing the Build Environment
+==================================
+
+Before you can use Yocto you need to setup the build environment.
+From within the ``poky`` directory, source the :ref:`ref-manual/structure:\`\`oe-init-build-env\`\`` environment
+setup script to define Yocto Project's build environment on your build host::
+    
+    $ source oe-init-build-env 
+
+Note, that this step will have to be repeated every time you open a new shell.