Update Readme to inform users they can use their Emma Credentials

[sastrakhan]

Users can utilize their Emma credentials instead of the private/public
key to access the API if the keys are unavailable

[kylegregory]

Hey @sastrakhan

Thanks for the PR. Where did you find this information? I'm not seeing that ability in the documentation: http://api.myemma.com/

[sastrakhan]

Hey @kylegregory. It's not listed in the documentation. I had a client requirement to use the username/password authentication and I was worried your authentication wouldn't support it but then I saw you were using the HttpBasicAuthentication object so I tried it and it worked.

I thought I'd suggest it as an addendum to the readme to inform developers before they tried an unnecessary path of making an alternative EmmaAPI constructor when it's not necessary ;)