feat(builder): select builder by adding DRYCC_BUILDER

Author: duanhongyi

src/applications/using-buildpacks.md

@@ -147,6 +147,15 @@ and set `BUILDPACK_URL` to the URL:
     $ drycc config:set BUILDPACK_URL=git@github.com:user/private_buildpack.git
     $ git push drycc master
 
+## Builder selector
+
+Which way to build a project conforms to the following principles:
+
+- If Dockerfile exists in the project, the builder uses `dockerbuilder`
+- If Procfile exists in the project, the builder uses `slugbuilder`
+- If both exist, dockerfile is used by default
+- You can also set the `DRYCC_BUILDER` to determine which builder to use.
+
 
 [pods]: http://kubernetes.io/v1.1/docs/user-guide/pods.html
 [controller]: ../understanding-workflow/components.md#controller