readme.md: wrote about kill

readme.md

@@ -17,13 +17,14 @@ Expect for Windows Powered by GopherLua
  - `send(S,MS)` waits MS [m-seconds] per 1-character (for plink.exe)
  - `sendln()` is same as send() but append CR.
  - `spawn()` starts applications and
- - On success, it returns PROCESS-ID
+ - On success, it returns PROCESS-ID(integer)
  - On failure, it returns nil
  - `echo()` controls echoback
  - `echo(true)`: echo on
  - `echo(false)`: echo off
  - `echo("...")`: print a string
  - `arg[]` contains commandline arguments (`arg[0]` is scriptname)
+ - `kill(PROCESS-ID)` kills the process.
 
 Sample
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