Update GCR IAM settings in Tekton development doc
Update GCR IAM settings in Tekton development doc

The current setting of GCR in the development doc does not include instructions to configure GCR IAM policies. This results in ErrImagePull issue when pulling images from GCR to run Tekton project (due to missing permission). This commits adds the missing IAM policy setup instructions under the using Google Container Registry (GCR) section.

tektoncd/pipeline,tektoncd/pipeline