Contributing guide updates (#239)

* Specify support for only latest and last major Grafana versions.
* List pre-installed Grafana plugins to indicate more specifically which
  are intended to be supported.
* Rename "Plugin Support" section to "Feature Support"

CONTRIBUTING.md

@@ -8,10 +8,47 @@ Grafana](https://github.com/grafana/grafana/blob/master/CODE_OF_CONDUCT.md). To
 help us create a safe and positive community experience for all, we require all
 participants to adhere to it.
 
-## Plugin Scope
+## Version Support
+
+We strive to only support the latest and last major Grafana versions - currently
+6 and 7.
+
+## Feature Support
 
 **Grafonnet only aims to support core features and plugins.**
 
+In terms of plugins, this means only those that are pre-installed:
+
+* Alert List
+* Azure Monitor
+* Bar gauge
+* CloudWatch
+* Dashboard list
+* Elasticsearch
+* Gauge
+* Graph
+* Graphite
+* Heatmap
+* InfluxDB
+* Jaeger
+* Logs
+* Loki
+* Microsoft SQL Server
+* MySQL
+* News
+* OpenTSDB
+* Plugin list
+* PostgreSQL
+* Prometheus
+* Singlestat
+* Stackdriver
+* Stat
+* Table
+* Table (old)
+* TestData DB
+* Text
+* Zipkin
+
 While Grafonnet itself only supports core Grafana functionality, we strongly
 encourage development and use of community Grafonnet extensions. See the
 [Community Plugins](https://grafana.github.io/grafonnet-lib/community-plugins/)