Docs: Updated roadmap issue to link to the pinned roadmap issues

ROADMAP.md

-# Roadmap (2018-08-07)
-
 This roadmap is a tentative plan for the core development team. Things change constantly as PRs come in and priorities change. 
 But it will give you an idea of our current vision and plan. 
   
-### Short term (1-2 months)
-  - PRs & Bugs
-  - React Panel Support
-  - React Query Editor Support
-  - Metrics & Log Explore UI 
-  - Grafana UI library shared between grafana & plugins
-  - Seperate visualization from panels
-  - More reuse between Explore & dashboard
-  - Explore logging support for more data sources 
-   
-### Mid term (2-4 months)  
-  - Drilldown links
-  - Dashboards as code workflows 
-  - React migration
-  - New panels 
-  
-### Long term (4 - 8 months)
- - Alerting improvements (silence, per series tracking, etc)
-
-### In a distant future far far away
- - Meta queries 
- - Integrated light weight TSDB
- - Web socket & live data sources
-
-### Outside contributions
-We know this is being worked on right now by contributors (and we hope to merge it when it's ready). 
-
+Go to the Issues tab on GitHub. There you will find, at the top, 3 pinned roadmap issues.