README: Add prominent section about plugins
README: Add prominent section about plugins

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