Ensure release version on README is not incorrect

[scottrigby]

Good fix for #20

On the main branch we now use a {{version}} placeholder at the top of PRINCIPLES.md markdown file, as a way to solve #20.

Incomplete though

However, we forgot to resolve this in the README 🤦, which still links to the first pre-release:

The current version v0.1.0 is a pre-release, which includes:

• GitOps Principles
• GitOps Glossary

Sigh.

Proposed solutions

1. change the README to use a placeholder
   • pros: the {{version}} => semver replacement could be automated, just as we plan to do for the PRINCIPLES.md on release branches
   • cons: users who encounter the readme would likely be confused by broken links on main
2. change the text above to just point to the list of releases and milestones for planned upcoming releases
   • pros: the README will be clear to new users. also don't have to update it for every new release
   • cons: README won't list the current version on main
3. solve README differently than PRINCIPLES.md, since the goal is different. On PRINCIPLES.md we don't want the previously published version to be displayed after new commits are added to main, because of Principles version in markdown may not match current revision viewed #20. However in the README we (may) want to list the most recently published version. Perhaps this could be solved by a GitHub action.

How about we start with option 2, and consider option 3 when we get around to making a release workflow using gh actions (similar to https://github.com/helm/chart-testing#releasing).

[lloydchang]

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