OM 2.0: Make it safe to develop on main

[dashpole]

Many things link to the main branch of this repo:

• openmetrics.io
• OTel spec
• Probably more

We should:

• Update links to link to the 1.0 release.
• Add warning links to the repository README and specification/OpenMetrics.md stating that it is under development, and link to OM 1.0.
• More?