Refactor README and add CONTRIBUTING instructions

Author: vraj152

CONTRIBUTING.md

@@ -0,0 +1,4 @@
+# How to Contribute
+
+Currently this project is not accepting contributions. Should you have any
+concerns, please open an issue and project maintainers will be in touch.

README.md

@@ -1,12 +1,17 @@
-This directory contains the VSAs for GKE VM images. The subfolder structure will follow the pattern -
+This directory contains the VSAs (Verification Summary Attestations) for GKE VM images.
+(we are not making VSAs for container images public at the moment). The subfolder
+structure will follow the pattern -
 
 ```
-<VM_IAMGE_PROJECT>/<VM_IMAGE_NAME>:<VM_IMAGE_ID>.intoto.jsonl
+<VM_IAMGE_PROJECT_ID>:<VM_IAMGE_PROJECT_NUMBER>/<VM_IMAGE_NAME>:<VM_IMAGE_ID>.intoto.jsonl
 ```
 
 For eg: VSA for -
-`gke-master-images:gke-1288-gke1166000-cos-109-17800-147-54-c-pre` will be stored at -
-`gke-master-images/gke-1288-gke1166000-cos-109-17800-147-54-c-pre:7627254251642584359.intoto.jsonl`
-where `7627254251642584359` is GCE VM image ID.
+`gke-master-images/gke-1288-gke1166000-cos-109-17800-147-54-c-pre` will be
+stored at -
+`gke-master-images:78064567238/gke-1288-gke1166000-cos-109-17800-147-54-c-pre:7627254251642584359.intoto.jsonl`
+where `78064567238` is the Project number & `7627254251642584359` is GCE VM image ID.
 
-* GitHub repo: https://github.com/googlecloudPlatform/gke-vsa
+* GitHub repo: https://github.com/googlecloudPlatform/gke-vsa
+
+*Note:* This directory does **not** exist yet.