Add an example to README

[masatake]

#include <readtags.h>
#include <stdio.h>

static void printEntry (tagEntry *entry)
{
  printf ("%s,%s,%lu,%s\n", entry->name,
	  entry->file, entry->address.lineNumber,
	  entry->address.pattern);
}
int
main(int argc, char **argv)
{
  char *tagfile = argv[1];
  if (tagfile == NULL)
    {
      fprintf(stderr, "Usage: %s TAGFILE [WORD]\n", argv[0]);
      return 1;
    }
  char *word_for_search = (argc < 2)? NULL: argv[2];
  tagFileInfo tinfo;
  
  tagFile *t = tagsOpen (tagfile, &tinfo);
  if (t == NULL)
    return 1;
  
  tagResult r;
  tagEntry entry;
  tagResult (*next) (tagFile *const, tagEntry *const);
  
  if (word_for_search)
    {
      next = tagsFindNext;
      r = tagsFind (t, &entry, word_for_search, 0);
    }
  else
    {
      next = tagsNext;
      r = tagsFirst (t, &entry);
    }

  if (r == TagSuccess) 
    {
      do
	{
	  printEntry (&entry);
	  r = (*next) (t, &entry);
	}
      while (r == TagSuccess);
    }
  tagsClose (t);
  return 0;
}