Document permissions setup for resources access by Dashboard

[jessm12]

Came across while investigating #212

Expected Behavior

Our docs should advise users how to setup the required access control to allow them to run the dashboard and see their Tekton resources in the UI

Actual Behavior

In order to view Tekton resources through the dashboard, the service account that it is running under must have the relevant permissions against it to access Tekton resources - for example the tekton-pipelines-controller has these permissions set against it, Clusterroles and Bindings.

When I was trying to get the dashboard up and running on Minishift following https://github.com/tektoncd/dashboard#install-on-minishift the default service account was used and no resources were displayed as it didn't have the permissions to get Tekton resources.

Steps to Reproduce the Problem

1. Install on Minishift or any environment using a service account that doesn't have the required permissions to access Tekton resources
2. Observe the the errors in fetching the resources

[CarolynMabbott]

/assign

[CarolynMabbott]

/status in-progress

[a-roberts]

@steveodonovan and I were discussing Openshift/Minishift permissions just now, he's pointed out his changes here should certainly be of use - I'd try running with said cluster role/binding etc

[steveodonovan]

@CarolynMabbott We might have some overlap here. This access is a prerequisite for the proxy. Complete in the PR below but still needs testing on openshift

https://github.com/tektoncd/dashboard/pull/268/files#diff-6ce617087fa9cd260ef0f682b7602619R1

[a-roberts]

I've tested this on OpenShift and all is well with roles I've added for the extension and that we've modified for the dashboard, gonna close this off, cheers all