Update readme to include instructions on how to deal with root CAs

[RandalliLama]

The list of root certificate authorities that node trusts is hard coded. The list is in this file:

https://github.com/nodejs/node/blob/master/src/node_root_certs.h

There is already an issue filed against node complaining about the fact that the list is hardcoded.

nodejs/node#4175

There is some code in one of the ADAL samples in this repo that indicate how to handle a CA that is not in the hard coded list.

azure-activedirectory-library-for-nodejs/sample/username-password-sample.js

Line 84 in 606d011

// +++++ Using a Private CA ++++++