Add Zero SSL documentation

docs/README.md

@@ -20,6 +20,8 @@
 
 [Standalone certificates](./Standalone-certificates.md)
 
+[Zero SSL](./Zero-SSL.md)
+
 #### Troubleshooting:
 
 [Invalid / failing authorizations](./Invalid-authorizations.md)

docs/Zero-SSL.md

@@ -0,0 +1,14 @@
+## Zero SSL
+
+[Zero SSL](https://zerossl.com/) is an ACME CA that offer some advantages over Let's Encrypt:
+- no staging endpoint and [no rate limiting on the production endpoint](https://zerossl.com/features/acme/).
+- web based [management console](https://zerossl.com/features/console/) to keep track of your SSL certificates.
+
+Using Zero SSL through an ACME client, like in this container, allows for unlimited 90 days and multi-domains (SAN) certificates.
+
+Unlike Let's Encrypt, Zero SSL requires the use of an email bound account. If you already created a Zero SSL account, you can either:
+
+- provide pre generated [EAB credentials](https://tools.ietf.org/html/rfc8555#section-7.3.4) using the `ACME_EAB_KID` and `ACME_EAB_HMAC_KEY` environement variables.
+- provide your ZeroSSL API key using the `ZEROSSL_API_KEY` environement variable.
+
+If you don't have a ZeroSSL account, you can let **letsencrypt-nginx-proxy-companion** create a Zero SSL account with the adress provided in the `ACME_EMAIL` or `DEFAULT_EMAIL` environment variable. Note that the adresse that will be used must be a valid email adress that you actually own.