With the release of Sitecore 8.1 a new Pipeline called getFieldLabel was added. This new pipeline allows you to inject code that can append or alter the label of a field within the Content Editor.
The first thing to notice is that the pipeline accepts a GetFieldLabelArgs class for pipeline. This contains the field in question and the result. The result is a string of what will be appended to the label.
Within the process method you have access to the field that your updating, the result string can be appended to by your code. This is what is displayed in the Content Editor above the field, it can be text or HTML.
Below is an example of what gets rendered in the Content Editor.