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

[hypesystem]

Fixes #1229

[mmalecki]

Just a really minor nitpick, but mind changing 'ca' to just ca? No apostrophes needed here.

[hypesystem]

I am following the convention laid out in previous examples.

From the first SSL/TLS example:

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 :)

[mmalecki]

lgtm, besides that minor nitpick!

[nylen]

I'm in favor of going ahead and removing the quotes in the other example too, but that's not a big deal to me.

More importantly, what does a custom CA actually do? I think users will want to see something like this:

To make request accept other SSL certificates (for example, self-signed certificates from your company), specify the certificate authority (CA) that signed the certificate by sending the contents of the authority's certificate file as ca:

Awkward wording, definitely could be done better.

Also, does specifying a custom CA override Node's built-in CAs? I think that's worth clarifying too.

[hypesystem]

I have tried to accomodate the requested changes (which are all very valid).

I dont know if the text is the best it gets, but the use case is now clearer.

It is possible to accept other certificates than those signed by generally allowed Certificate Authorities (CAs). This can be useful, for example, when using self-signed certificates. To allow a different certificate, you can specify the signing CA by adding the contents of the CA's certificate file to the agentOptions:

[nylen]

Looks good to me, thanks @hypesystem!