fix: update README to point to the OLM book website

[exdx]

Description of the change:

Motivation for the change:
A new user coming to the OLM repo and reading the README would be interested in the OLM book. The link we provide should point to the OLM book itself, not the git repo backing the content. This makes for a better user experience for someone looking into OLM for the first time.

Reviewer Checklist

• Implementation matches the proposed design, or proposal is updated to match implementation
• Sufficient unit test coverage
• Sufficient end-to-end test coverage
• Docs updated or added to /docs
• Commit messages sensible and descriptive

[Bowenislandsong]

should we put https://olm.operatorframework.io/ on there?

[awgreene]

/lgtm

[openshift-ci-robot]

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

Test name	Commit	Details	Rerun command
ci/prow/images	3493ed1	link	/test images
ci/prow/e2e-gcp	3493ed1	link	/test e2e-gcp
ci/prow/e2e-gcp-upgrade	3493ed1	link	/test e2e-gcp-upgrade
ci/prow/e2e-aws-olm	3493ed1	link	/test e2e-aws-olm

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.

[njhale]

/retest
/approve

[openshift-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: exdx, njhale

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [njhale]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[exdx]

will manually merge as this is doc-only (only updates a link in the README).