Mention that 1.x is feature-complete

It'll help would-be contributors (such as myself, see chartjs#2058) avoid creating PRs that won't be accepted.

Author: aaron-em

CONTRIBUTING.md

@@ -35,6 +35,8 @@ Pull requests
 
 Clear, concise pull requests are excellent at continuing the project's community driven growth. But please review [these guidelines](https://github.com/blog/1943-how-to-write-the-perfect-pull-request) and the guidelines below before starting work on the project.
 
+Be advised that **Chart.js 1.0.2 is in feature-complete status**. Pull requests adding new features to the 1.x branch will be disregarded.
+
 Guidlines:
 
  - Please create an issue first: