describe ClusterBuildTemplate and its use within a Build (#604)

build/build-templates.md

@@ -7,8 +7,12 @@ A set of curated and supported build templates is available in the
 
 ## What is a Build Template?
 
-A `BuildTemplate` encapsulates a shareable [build](./builds.md) process with
-some limited parameterization capabilities.
+A `BuildTemplate` and `ClusterBuildTemplate` encapsulates a shareable [build](./builds.md)
+process with some limited parameterization capabilities.
+
+A `BuildTemplate` is available within a namespace, and `ClusterBuildTemplate` is available across entire Kubernetes cluster.
+
+A `BuildTemplate` functions exactly like a `ClusterBuildTemplate`, and as such all references to `BuildTemplate` below are also describing `ClusterBuildTemplate`.
 
 ### Example template
 
@@ -95,6 +99,7 @@ spec:
       revision: master
   template:
     name: dockerfile-build-and-push
+    kind: BuildTemplate
     arguments:
       - name: IMAGE
         value: gcr.io/my-project/rester-tester
@@ -110,6 +115,7 @@ spec:
       revision: master
   template:
     name: dockerfile-build-and-push
+    kind: BuildTemplate
     arguments:
       - name: IMAGE
         value: gcr.io/my-project/wget
@@ -128,6 +134,7 @@ spec:
       revision: master
   template:
     name: dockerfile-build-and-push
+    kind: BuildTemplate
     arguments:
       - name: IMAGE
         value: gcr.io/my-project/docker
@@ -138,6 +145,8 @@ spec:
         value: Dockerfile-17.06.1
 ```
 
+The `spec.template.kind` is optional and defaults to `BuildTemplate`. Alternately it could have value `ClusterBuildTemplate`.
+
 ---
 
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