doc: clarify "Reviewed-By" iff "LGTM"

As per conversation with @Trott, make it clear that Reviewed-By lines
should only be added for collaborators who've actually put a LGTM on the
PR.

PR-URL: https://github.com/nodejs/node/pull/7183
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: JacksonTian - Jackson Tian <shvyo1987@gmail.com>

doc/onboarding.md

@@ -169,6 +169,7 @@ Landing a PR
     * `Reviewed-By: human <email>`
       * Easiest to use `git log` then do a search
       * (`/Name` + `enter` (+ `n` as much as you need to) in vim)
+      * Only include collaborators who have commented "LGTM"
     * `PR-URL: <full-pr-url>`
 * `git push upstream master`
     * close the original PR with "Landed in `<commit hash>`".