Add a description field to pipeline workspace declarations

A description field can help rationalize how a workspace will be used
across multiple tasks in a pipeline without forcing the user to read and
interpret the tasks themselves.

This PR adds an optional description field to pipeline workspace
declarations to help with this.

pkg/apis/pipeline/v1alpha2/workspace_types.go

@@ -77,6 +77,11 @@ type WorkspaceBinding struct {
 type WorkspacePipelineDeclaration struct {
 	// Name is the name of a workspace to be provided by a PipelineRun.
 	Name string `json:"name"`
+	// Description is a human readable string describing how the workspace will be
+	// used in the Pipeline. It can be useful to include a bit of detail about which
+	// tasks are intended to have access to the data on the workspace.
+	// +optional
+	Description string `json:"description"`
 }
 
 // WorkspacePipelineTaskBinding describes how a workspace passed into the pipeline should be