dev-manual: Updated the "Core System Development" section

dev-manual: Updated the "Core System Development" section

Added a new bullet as a tip for best practices that addresses
how we address bug 3071 and 3274.  We are suggesting they enable
a PR Service if they are using continually incrementing PR values
when package feeds are published or used.

Reported-by: Richard Purdie <richard.purdie@linux.com>
(From yocto-docs rev: b90497f139b1b6f7ab5510313eb297d5d7fb4e68)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>