MAHOUT-2211 Add Roadmap to Readme.md

[rawkintrevo]

Purpose of PR:

Add roadmap to readme.md

Important ToDos

Please mark each with an "x"

• [x] A JIRA ticket exists (if not, please create this first)[https://issues.apache.org/jira/browse/mahout/]

• [x] Title of PR is "MAHOUT-XXXX Brief Description of Changes" where XXXX is the JIRA number.

• [x] Created unit tests where appropriate

• [x] Assigned JIRA to self

Does this change break earlier versions? No

Is this the beginning of a larger project for which a feature branch should be made? No