Add `/directory` to example `NewClient` caDirURL. (#471)
Add `/directory` to example `NewClient` caDirURL. (#471)

The `acme.NewClient` function's `caDirURL` argument is expected to be
the full path to the ACME server's directory endpoint. In the README
example of using Lego programmatically against a Boulder instance only
the hostname & port are provided but not the directory path:

   `"http://192.168.99.100:4000"`

This produces an error like:

```
2018/01/15 14:34:06 get directory at 'http://192.168.99.100:4000': invalid
character '<' looking for beginning of value
```

When used verbatim with a Boulder container since the `/directory` is
missing and "What is an ACME server" HTML index page is returned.

This commit updates the example to use:

 `"http://192.168.99.100:4000/directory"`

Which allows the example code to work with Boulder as-intended.
xenolf/lego,xenolf/lego