x/website: add link to cmd/golangorg/README.md from README.md

[julieqiu]

It's not immediately clear from the top-level README how to run cmd/golangorg, but this is explained in cmd/golangorg/README.md.

We should add a short description and link to cmd/golangorg/README.md from the top-level README.

See https://github.com/golang/build#overview as an example.

/cc @dmitshur @AceroM

[gopherbot]

Change https://golang.org/cl/251121 mentions this issue: website: update README.md