pulseIn() pullup/down comment

[russmcinnis]

There is a situation where using pulseIn() on a pin that has a floating voltage could cause a sketch to hang at the pulseIn() function call. A comment was added to the pulseIn() function header to use a pull up or pull down resistor.

[russmcinnis]

@SidLeung please review.

[eriknyquist]

Looks good to me, @SidLeung to review also

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}