introduction.rst: add a note about eSDKs

The extensible SDK (eSDK) is an important piece of
the overall development story. Therefore, a note about its
existence makes sense in the Reference Kit introduction.rst too.

To get started with the eSDK in refkit, we primarily rely
on the comprehensive Yocto Project SDK manual. The SDK manual
provides the necessary information how to build the eSDK
installer for refkit profiles/images.

Completes [YOCTO: #11493].

Signed-off-by: Mikko Ylinen <mikko.ylinen@linux.intel.com>

Author: mythi

doc/introduction.rst

@@ -27,6 +27,15 @@ By default the Reference Kit runs on Intel's `meta-intel BSP`_ platforms.
 .. _`meta-intel BSP`: https://www.yoctoproject.org/product/meta-intel-bsp-layer
 
 
+The `Yocto Project extensible software development kit`_ (eSDK) is comes
+pre-configured and pre-tested in Reference Kit repository but the users of
+it must build the SDK installer themselves. By default, the eSDK type is
+"minimal" which implies at least the ``SSTATE_MIRROR`` setting must be adjusted
+when building distributable eSDKs. For more documentation on how to customize and
+re-configure the eSDK, see the "Appendig B. Customizing the Extensible SDK"
+in the `Yocto Project extensible software development kit`_ manual.
+
+.. _`Yocto Project extensive software development kit`: http://www.yoctoproject.org/docs/latest/sdk-manual/sdk-manual.html
 .. _`Yocto Project release cadence`: https://wiki.yoctoproject.org/wiki/Planning#Roadmaps_and_Schedules
 .. _`Yocto Project Bugzilla`: https://bugzilla.yoctoproject.org/
 .. _`Yocto Project git`: http://git.yoctoproject.org/