added volume-cache plugin

[mjwwit]

Added drone-volume-cache plugin to the plugin list.

[bradrydzewski]

I'm going back and forth on how I want to handle docs, and whether or not we should keep them distributed (ie download from repos) or central. The pros of central is that we have a bit more control over a standard template, style, translations, etc. The negative is that it probably won't scale if / when we have hundreds of plugins.

Right now I'm leaning toward centralized, where all plugin docs use this archetype:
https://github.com/drone/drone-plugin-index/blob/master/archetypes/default.md

I took your docs and added to the above archetype template, which now looks like this:
http://plugins.drone.io/drillster/drone-volume-cache/

I may decide in another week or so to reverse this decision, and if so, will send you a PR for the new 0.5 formatted DOCS.md

[mjwwit]

Looks great!
We'll make it work with whatever format you decide is best 😄