Dashboard: Dev Guide

[wbuchwalter]

#411
Add a developer guide for the dashboard.

More doc is ultimately needed, but this is the most pressing part to unblock people from contributing.

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This change is 

[k8s-ci-robot]

@wbuchwalter: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
kubeflow-tf-operator-presubmit	2ccfafa	link	/test kubeflow-tf-operator-presubmit

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[coveralls]

Coverage remained the same at 27.475% when pulling 2ccfafa on wbuchwalter:doc-dashboard into 0759f7a on kubeflow:master.

[gaocegege]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: gaocegege

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[gaocegege]

@jlewi It seems that we met an error when CI setups a cluster:

http://testing-argo.kubeflow.io/timeline/kubeflow-test-infra/kubeflow-tf-operator-presubmit-tfjob-e2e-417-2ccfafa-106-72ee;tab=workflow;node=kubeflow-tf-operator-presubmit-tfjob-e2e-417-2ccfafa-106-72ee-2969931507:logs

[jimexist]

@wbuchwalter i can help adding sections about linting, etc. if needed.

[gaocegege]

Merge it since it is about docs, then no need to be blocked by the CI.