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