Add conspicuous and concise explanation about the nature of the opam-repository

[shonfeder]

This issue is to address @ejgallego's question at Deducteam/lambdapi#1229 (comment)

While there is existing documentation about packaging, publishing packages, and working with the CI results on package publication PRs, there is not a single, concise, high-level explanation of how the opam-repository works from a package author perspective, and why it works as it does.

Here is the existing documentation I know of, along with a note about why it doesn't meet the stated need:

• https://github.com/ocaml/opam-repository/tree/master/governance/policies -- This specifies the policies, and explains their rational, but it is meant to serve enforcement and regulation, not to communicate the overall sense and purpose.
• https://github.com/ocaml/opam-repository/blob/master/CONTRIBUTING.md -- This provides HOWTO documentation for publishing packages via different approaches, but it does not explain basic concepts, like the fact that the repository is curated.
• https://github.com/ocaml/opam-repository/wiki/How-to-deal-with-CI -- As the title suggests, this explains how to deal with the CI, but does not offer a high-level of account of why the CI runs and how package authors and users benefit from it.

What we are lacking, AFAICT, is a single, highly visible and easily accessible document that covers the following points:

• How and why opam-repository is curated.
• How and why package publication is checked by CI (e.g., along the lines of The ok_ko test takes an unreasonable amount of RAM Deducteam/lambdapi#1229 (comment)
• Sketch a very high-level view of what is involved in publication and maintenance

I think we can adequately explain each of these points in one or two short paragraphs, and then link to the more detailed documentation noted above.

I would propose that this kind of high-level explanatory material would be a good fit for a section on the [./README.md][https://github.com/ocaml/opam-repository/blob/master/README.md].