Add documentation re: getting the stdout to clipboard on OS X and any *nix systems with support

Adds simple support for piping Markdown or text to clipboard for simplification of copy/paste needs (e.g. pasting into a Github PR or IR thread).

### OS X

```
$ ufodiff ... | pbcopy
```

### .nix Support

https://stackoverflow.com/questions/749544/pipe-to-from-the-clipboard?noredirect=1 

Possibly Windows support too suggested in above SO thread