Document operator-sdk up local

[fanminshi]

The operator-sdk up local is added recently to improve the developer's local development workflow. Essentially when operator-sdk up local --kubeconfig is called, you can launch your operator locally and your operator can talk to a kubernetes cluster via passed in kubeconfig; Hence, you bypass the phases where building/pushing docker image and improve the speed of development/testing.

However, this is no documentation on how to use it unless you read the PR #219.

It will be great if we can write a doc about how to use this command. I think it can benefits the users greatly.

See #219 and #142 to learn about what this command is about.

[fanminshi]

Additional Task:

• Once operator-sdk up local failed to run #273 is fixed, we need to update the user_guide with the fix. ref: doc: modify user_guide with watched namespace fixed #279