docs: dev guide for en missing GOPATH setup

[backsapce]

Ⅰ. Describe what this PR does

dev guide for en missing GOPATH setup

Ⅱ. Does this pull request fix one issue?

fixes #XXXX nothing issue fixed

Ⅲ. List the added test cases (unit test/integration test) if any, please explain if no tests are needed.

just docs improvment, no tests needed

Ⅳ. Describe how to verify it

run cmd follow by get start guid

Ⅴ. Special notes for reviews

[fluid-e2e-bot[bot]]

Hi @backsapce. Thanks for your PR.

I'm waiting for a fluid-cloudnative member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.

[fluid-e2e-bot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cheyang

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/fluid-cloudnative/fluid/blob/master/OWNERS)~~ [cheyang] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment