Add doc about enabling SSL and basic auth

This addresses part of issue #432.
Link to local Marathon site in README

Make it apparent the Marathon site is served at “/marathon”.
Clarify SSL configuration steps

* Use “.pem” extension for certificate since it is the common format
  received from certificate authorities.
* Use “name” and “srcalias” to support multiple certs in PKCS format.
* Append “.jks” extension for “Java Keystore”.
Add missing 'n' to 'unencrypted'
Link to HTTPS version of Marathon as final step
Refer to Java keystore as standard “.jks”
Use env variables for passwords

This makes automation easier and assigns meaningful names to passwords.
Use env variable for all passwords
Merge pull request #484 from mesosphere/ssl-basic-auth-doc

Add doc about enabling SSL and basic auth