Document response file handling (emcc @filename.txt)

perhaps something like this from the gcc man page:

```
@file
Read command-line options from file. The options read are inserted in place of the original @file option. If file does not exist,
or cannot be read, then the option will be treated literally, and not removed.

Options in file are separated by whitespace. A whitespace character may be included in an option by surrounding the entire option
in either single or double quotes. Any character (including a backslash) may be included by prefixing the character to be included
with a backslash. The file may itself contain additional @file options; any such options will be processed recursively.
```

We probably want to make sure we actually implement it like this too.. or at least define our behaviour.