This achievement is super easy to get and there's already a video up here, but I wanted to at least just get a screenshot if people want to get a quick look at the exact location!
It's found almost in plain sight on a counter inside Kendo Gun Store directly after leaving the RPD in Leon's campaign.
For those wanting to know what it says:
*** Spoiler - click to reveal ***"Letter to the Shopkeeper
Kendo,
It seems like you've got your reasons for staying put, so I'll trust you to take care of yourself.
Still, if things get worse and your "other arrangements" don't work out, you know where to find me. I'll do everything I can.
Jill Valentine"