site/plugins: add developing guide
site/plugins: add developing guide

This path adds a guide with code examples based on the Scope IOWait plugin
on how to develop a plugin for scope.

weaveworks/scope,alban/scope,alban/scope,alban/scope,alban/scope,paulbellamy/scope,alban/scope,kinvolk/scope,kinvolk/scope,paulbellamy/scope,kinvolk/scope,kinvolk/scope,paulbellamy/scope,weaveworks/scope,weaveworks/scope,paulbellamy/scope,weaveworks/scope,kinvolk/scope,paulbellamy/scope,kinvolk/scope,weaveworks/scope,weaveworks/scope,alban/scope,paulbellamy/scope