Add CONTRIBUTING.md.
Add CONTRIBUTING.md.

mozilla/nunjucks,mozilla/nunjucks,internalfx/nunjucks,oddbird/nunjucks,devoidfury/nunjucks,atorkhov/nunjucks,internalfx/nunjucks,internalfx/nunjucks,mozilla/nunjucks,devoidfury/nunjucks,atorkhov/nunjucks,oddbird/nunjucks,internalfx/nunjucks,kevinschaul/nunjucks,atorkhov/nunjucks,robgraeber/nunjucks,oddbird/nunjucks,kevinschaul/nunjucks,devoidfury/nunjucks,robgraeber/nunjucks,kevinschaul/nunjucks,oddbird/nunjucks,robgraeber/nunjucks,robgraeber/nunjucks,kevinschaul/nunjucks,atorkhov/nunjucks