doc: added a note on pulling task image from private repo

Prior to this, docs were not clear on how to use a private image
for a task - issue tektoncd#3604

If a task uses an image from a private repo, then `imagePullSecrets`
must be provided at runtime via `podTemplate` - added note to the tasks docs

docs/tasks.md

@@ -175,6 +175,9 @@ The following requirements apply to each container image referenced in a `steps`
   though Tekton runs `Steps` sequentially.
   For more detail, see [Compute Resources in Tekton](./compute-resources.md).
 
+**Note:** If the image referenced in the `step` field is from a private registry, `TaskRuns` or `PipelineRuns` that consume the task
+          must provide the `imagePullSecrets` in a [podTemplate](./podtemplates.md).
+
 Below is an example of setting the resource requests and limits for a step:
 
 ```yaml