doc: ngx.pipe: mentioned lua-resty-signal's signum() function.

Signed-off-by: Thibault Charbonnier <thibaultcha@me.com>

Author: spacewander

lib/ngx/pipe.md

@@ -283,6 +283,11 @@ Sends a signal to the sub-process.
 Note that the `signum` argument should be signal's numerical value. If the
 specified `signum` is not a number, an error will be thrown.
 
+You should use [lua-resty-signal's signum()
+function](https://github.com/openresty/lua-resty-signal#signum) to convert
+signal names to signal numbers in order to ensure portability of your
+application.
+
 In case of success, this method returns `true`. Otherwise, it returns `nil` and
 a string describing the error.