Document how to use custom CA in README (#1229)

Fixes #1229
I am following the convention laid out in previous examples.

From the first SSL/TLS example:

```javascript
var options = {
    url: 'https://api.some-server.com/',
    agentOptions: {
        'cert': fs.readFileSync(certFile),
        'key': fs.readFileSync(keyFile),
        // Or use `pfx` property replacing `cert` and `key` when using private key, certificate and CA certs in PFX or PKCS12 format:
        // 'pfx': fs.readFileSync(pfxFilePath),
        'passphrase': 'password',
        'securityOptions': 'SSL_OP_NO_SSLv3'
    }
};
```

This could definitely be corrected, but I don't think a change of style has anything to do with this pull request :)