Document instrumentation hosting recommendations.

[MrAlias]

The size of the developer community supporting this project is not large enough to support the ever growing amount of instrumentation being submitted to this repository. We need to document instrumentation guidelines discussed in last week's SIG meeting that address this with the following prioritization of where instrumentation should be hosted:

1. Native to any package. Instrumentation exists in the package itself.
2. Hosted by the maintainer in their own public git repository. List it in the opentelemetry.io registry.
3. Hosted by OTel in contrib. We need to document when this is warranted and what the process will be to submit.

[MrAlias]

As a follow on to documenting the project policy, we need to close outstanding PRs here for instrumentation we will not be accepting.