Adds buildah template creation step in README

[navidshaikh]

This change adds step in README to create buildah build-template.

[vdemeester]

/ok-to-test

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[navidshaikh]

/retest

[knative-prow-robot]

New changes are detected. LGTM label has been removed.

[knative-prow-robot]

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

Test name	Commit	Details	Rerun command
pull-knative-build-templates-build-tests	fe8caddd82b8ba350b19b9b8c5c6539d6c380c0c	link	/test pull-knative-build-templates-build-tests

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[navidshaikh]

The new update to patch includes fixes for errors caught while linting the markdown files as part of CI. Errors: ref logs, excerpts below

I0319 05:23:53.861] ---- Linting the markdown files ----
I0319 05:23:53.861] ------------------------------------
I0319 05:23:56.429] buildah/README.md:11: MD022 Headers should be surrounded by blank lines
I0319 05:23:56.430] buildah/README.md:12: MD031 Fenced code blocks should be surrounded by blank lines