Create PLUGIN_DEV.md

initial draft of plugin dev document

PLUGIN_DEV.md

+# Plugin Development 
+
+This document is not meant as complete guide for developing plugins but more as a changelog for changes in
+Grafana that can impact plugin development. When ever you as plugin author encounter an issue with your plugin after
+upgrading Grafana please check here before creating an issue. 
+
+## Links
+
+- [Datasource plugin written in typescript](https://github.com/grafana/typescript-template-datasource)
+- [Simple json dataource plugin](https://github.com/grafana/simple-json-datasource)
+- [Plugin development guide](http://docs.grafana.org/plugins/developing/development/)
+
+## Changes in v4.6
+
+This version of Grafana has big changes that will impact a limited set of plugins. We moved from systemjs to webpack
+for built-in plugins & everything internal. External plugins still use systemjs but now with a limited 
+set of Grafana components they can import. Plugins can depend on libs like lodash & moment and internal components 
+like before using the same import paths. Hover since everything in Grafana is no longer accessable some limited 
+set of plugins could encounter issues when importing a Grafana dependency. 
+
+[List of exposed components plugins can import/require](https://github.com/grafana/grafana/blob/master/public/app/features/plugins/plugin_loader.ts#L48)
+
+If you think we missed exposing a crucial lib or Grafana component let us know by opening an issue.  
+
+### Depricated components 
+
+The angular directive `<spectrum-picker>` is no depricated (will still work for a version more) but we recommend plugin authors
+to upgrade to new `<color-picker color="ctrl.color" onChange="ctrl.onSparklineColorChange"></color-picker>`
+