doc: add a sentence about REPLACEME in code changes

When adding to the REPL API recently, I was unsure what to put as a
version number for the new method I added. This change adds a reference
to `releases.md` where the use of `REPLACEME` is discussed.

PR-URL: https://github.com/nodejs/node/pull/25961


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>