Improve readability of HACKING doc

To keep readers from assuming step 3 is an incomplete sentence,
this patch changes the text from "Read on" to "See specific
guidelines below".  See comments[1] for more info.

[1] https://review.openstack.org/#/c/133258/3/HACKING.rst

Change-Id: I6f674c8661485994496f492c82152c7d0aba8ae7
Merge "Improve readability of HACKING doc"