Add document about copy command in makefile of examples.

[GreyZhang]

When type make error was reported : copy command not found.

What is the 'copy' command? Does it belong to a specified specified software? What is that?
I don't think it is just a alias of cp.

Perhaps this is a low-level issue, but it does make me puzzled. I hope to add more explanation.

[GreyZhang]

Ok, this should just be a alias of cp. So why not use cp instead?

[nrnrnr]

copy was a program written by Eric Roberts in the 1980s or earlier. It's meant to do roughly the same things is cp -ua but was written before cp necessarily had those options.

Usage: copy [-options] file1 file2
       copy [-options] file ... directory
Options:
  -f    Force copies of everything, ignoring dates
  -F    Force copies even if source is a link
  -l    Follow links to their targets before copy
  -v    Verbose (indicate all copies)
  -t    Trace (don't execute, but verbosely say what would be done)
  -R    Remove files in target not in the source
  -C    Note files that would be removed with -R
  -2    Force the "copy file1 file2" interpretation
  -n    Like -v, but don't make any copies
  -0300 Source file timestamps are three hours earlier than local time
  +0100 Source file timestamps are one hour later than local time
  -gmt  Source file timestamps are UCT
  -uct  Source file timestamps are UCT
  -xfile  (NO SPACE!) don't copy files or directories named `file'
  -~   Don't copy files whose names end in ~