Add a link to the Workspaces doc from docs/README.md

We recently introduced a workspaces doc to help consolidate
the disparate info spread across the other docs.

This commit adds a link to our docs/README.md index to
help surface workspace information. Further, it reorders
the index just a bit so that Workspaces & PipelineResources
appear next to each other without pushing the PipelineRun
doc further down in the list.

docs/README.md

@@ -58,8 +58,9 @@ See the following topics to learn how to use Tekton Pipelines in your project:
 - [Creating a Task](tasks.md)
 - [Running a standalone Task](taskruns.md)
 - [Creating a Pipeline](pipelines.md)
-- [Creating PipelineResources](resources.md)
 - [Running a Pipeline](pipelineruns.md)
+- [Defining Workspaces](workspaces.md)
+- [Creating PipelineResources](resources.md)
 - [Configuring authentication](auth.md)
 - [Using labels](labels.md)
 - [Viewing logs](logs.md)