The note can be found at Camp Venture, on the counter of the building at the weapons training range.
Remember how crazy you said I was to join up with the Brotherhood of Steel at Camp Venture? How you said I wouldn't be able to hack it, that I'd quit within a week? Well I'm writing to tell you that you were dead wrong. This is where I belong, side-by-side with people who give a crap about this world. Senior Knight Wilson treats me like I'm important, like I matter. Something you never did. I've found my calling, and I'm not coming home. The Brotherhood is my new family now. Now who's laughing?