conf-notes.txt: add shell environment header

The header text has been moved to meta/conf/conf-notes.txt by:
[ meta: move some text from oe-setup-builddir to conf-notes.txt ]

Need do the same to meta-poky/conf/conf-notes.txt.

(From meta-yocto rev: 8a9d72abcf484fbe8001a08623910c5af1be07a0)

Signed-off-by: Ming Liu <peter.x.liu@external.atlascopco.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta-poky/conf/conf-notes.txt

@@ -1,3 +1,8 @@
+
+### Shell environment set up for builds. ###
+
+You can now run 'bitbake <target>'
+
 Common targets are:
     core-image-minimal
     core-image-sato