feature: https.acme_staging takes Caddy's certificates from Let's Encrypt's staging environment.

README.md

@@ -141,6 +141,7 @@ must have the type shown below; an unknown section or setting is an error.
 | `https` | `email` | string | | The e-mail address of the Let's Encrypt account |
 | `https` | `web_port` | integer | `7444` | HTTPS port of the status web service |
 | `https` | `manager_port` | integer | `7443` | HTTPS port of the Router Manager |
+| `https` | `acme_staging` | boolean | `false` | Caddy takes its certificates from Let's Encrypt's staging environment alone, whose certificates no browser trusts and whose rate limits are higher, for routers rebuilt often (see HTTPS) |
 | `https` | `caddy_source` | string | `ubuntu` | Where Caddy is installed from: `ubuntu`, the Ubuntu archive, or `upstream`, Caddy's own repository (see HTTPS) |
 | `oidc` | `enabled` | boolean | `false` | The OpenID Provider at the status web service's HTTPS site, which needs `web.enabled`, `https.enabled` and `wan.local_fqdn` (see OpenID Provider) |
 | `sites` | `enabled` | boolean | `false` | Caddy serves the web sites (see Web sites) |
@@ -1471,7 +1472,17 @@ while metrics are enabled the metrics service on `metrics.https_port` (9465), al
 and renews the certificate from Let's Encrypt with the HTTP challenge, so `wan.local_fqdn` must
 resolve to the router's public address and port 80 must be reachable from the internet; the
 certificate and the ACME account are kept in `/var/lib/caddy`. Port 80 answers the challenge only;
-it does not redirect to HTTPS.
+it does not redirect to HTTPS. Caddy's default CAs are Let's Encrypt and ZeroSSL, which it takes
+when Let's Encrypt refuses, such as for its rate limit of 5 certificates a week for the same name.
+
+While `https.acme_staging` is `true`, Caddy's only CA is Let's Encrypt's staging environment, for
+routers that are rebuilt often. Its rate limits are far higher, but no browser or client trusts
+its certificates, so they are for tests only. The setting applies to every certificate Caddy
+obtains from an ACME CA, those of the web sites with `tls` `acme` included.
+
+```sh
+vpn-config-edit set --restart https.acme_staging true
+```
 
 `vpn-router-setup` writes Caddy's configuration to `/etc/caddy/vpn-router.Caddyfile` (root, group
 `caddy`, the group Caddy runs as, mode 0640), which the