doc: add performance notes for fs.readFile

Issue https://github.com/nodejs/node/issues/25741

 discusses a number of
performance considerations for fs.readFile, which was changed in Node.js
10.x to split discreet chunk reads over multiple event loop turns. While
the fs.readFile() operation is certainly slower than it was pre 10.x,
it's unlikely to be faster. Document the performance consideration and
link back to the issue for more in depth analysis.

Signed-off-by: James M Snell <jasnell@gmail.com>
Fixes: https://github.com/nodejs/node/issues/25741

PR-URL: https://github.com/nodejs/node/pull/36880


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>