Fix plugins docs
Fix plugins docs

* fix json highlighting on github
* added screenshot
* note about kernel version for bcc plugin
* sentence about seeing plugin in the UI

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