docs: building and readme outdated

hey there,

i did playing around with certmgr and cfssl to build internal CA for my homelab, and my first impression is : wow, what great toolset!
but i got a bit confusing while reading docs and making own experience with certmgr.

Can anybody explain how to build the binary of certmgr with go tools?! i am using golang 1.18.2.
```go get``` is deprecated and replaced by ```go install```. but ```go install github.com/cloudflare/certmgr/cmd/...@latest``` only build version 1.6.5 how to see [here](https://pkg.go.dev/github.com/cloudflare/certmgr).

since i am using the newest version, the "ca" key in my specs.json works fine and a deprecated info will show if i use the "ca" setting under "authority" like here:
https://github.com/cloudflare/certmgr/blob/4af0d7701d7500080ddc53bebb9c3d85977df3e4/README.md?plain=1#L152-L156
so is there a reason why it still in readme.md at the newest version?!
it confused me while get started with this really nice peace of application.

i think the docs thing is not much important, but how to build can help me and other people a lot. hoping for a short explanation.

Chris

Start with the README section linked in the issue and verify the current Go 1.18.2 installation behavior for certmgr, including go install github.com/cloudflare/certmgr/cmd/...@latest. Update the build instructions and clarify whether the deprecated authority.ca setting should remain documented; done means a newcomer can build the intended version and follow the current CA configuration guidance.

Written by the indexing model from the issue text.