Addjira

- Add a section on README to show where to check the status of old issues entered, where 
- Add a recommendation of entering new issues/feedback
Ticket created:  [SparkR-77](https://sparkr.atlassian.net/browse/SPARKR-77)
I'm not sure we'd want to say "Since this is incubating Spark subproject" at this time since such a phrase probably is widely interpreted as an official alpha component in Spark's mainline repo (e.g. Spark SQL, GraphX). 

How about: 
```
For better tracking and collaboration, issues and TODO items are reported to a dedicated [SparkR JIRA](https://sparkr.atlassian.net/browse/SPARKR/).
```