Add documentation for dependency installation feature

[yashjakhotiya]

What this PR does / why we need it: Without documentation, users might struggle in installation of additional python packages in the built docker image required for fairing jobs. This PR mentions the special role of 'requirements.txt' in input_files list of ml_tasks.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #NONE

Special notes for your reviewer:

This is a documentation update PR

Release note:

NONE

[kubeflow-bot]

This change is 

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[k8s-ci-robot]

Hi @yashjakhotiya. Thanks for your PR.

I'm waiting for a kubeflow 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.

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[yashjakhotiya]

/assign @jinchihe

[jinchihe]

/ok-to-test

[jinchihe]

/retest

[yashjakhotiya]

@jinchihe this is only a documentation update. Ideally, this should not fail any presubmit test.

[jinchihe]

@jinchihe this is only a documentation update. Ideally, this should not fail any presubmit test.

Yeah, that should be caused by test tenv, I will take a look later once get chance. Thanks.

[jinchihe]

/retest

[k8s-ci-robot]

@yashjakhotiya: The following test failed, say /retest to rerun all failed tests:

Test name	Commit	Details	Rerun command
kubeflow-fairing-presubmit	0505b25	link	/test kubeflow-fairing-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.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository. I understand the commands that are listed here.

[jinchihe]

@yashjakhotiya The CI testing problem has been fixed, would you please fetch and rebase? thanks a lot!