x/build: create instructions for contributor builders under LUCI

[cagedmantis]

We are shifting over to using LUCI as our build environment. The use of LUCI changes how the builders should be configured to connect to the LUCI platform. These instructions should be created and shared with the community.

@golang/release

[gopherbot]

Change https://go.dev/cl/504531 mentions this issue: cmd/genswarmcert, buildenv: create genswarmcert

[gopherbot]

Change https://go.dev/cl/504821 mentions this issue: cmd/bootstrapswarm: add bootstrapswarm used to bootstrap swarming bot

[cagedmantis]

https://github.com/golang/go/wiki/DashboardBuilders has been updated with the instructions. It is still being edited. The initial set of work has been completed.