README: update/fix contribution guidelines

- Ask to CC the docs@lists.yoctoproject.org mailing list
- doc/README: fix the command to generate the manual

(Bitbake rev: 8332664f9141d2c12f70589ebd2eed7eeddd8f77)

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/README

@@ -26,6 +26,11 @@ The git command to do that would be:
 
     git send-email -M -1 --to bitbake-devel@lists.openembedded.org
 
+If you're sending a patch related to the BitBake manual, make sure you copy
+the Yocto Project documentation mailing list:
+
+    git send-email -M -1 --to bitbake-devel@lists.openembedded.org --cc docs@lists.yoctoproject.org
+
 Mailing list:
 
     https://lists.openembedded.org/g/bitbake-devel