docs: add Google Trust Services instructions (#1144)

* Create Google-Trust-Services.md

Add basic details on using the GTS ACME endpoint.

* Update README.md

add a link to the Google Trust Services doc.

docs/Google-Trust-Services.md

@@ -0,0 +1,17 @@
+## Google Trust Services
+
+[Google Trust Service](https://pki.goog/) is an ACME CA with generous default quota and high ubiquity. 
+
+Using Google Trust Services through an ACME client, like in this container, allows for unlimited 90 days and multi-domains (SAN) certificates.
+
+### Activation
+
+Google Trust Services support is activated when the `ACME_CA_URI` environment variable is set to the Google Trust Services ACME endpoint (`https://dv.acme-v02.api.pki.goog/directory`).
+
+### Account
+
+Google Trust Services requires the use of an externally bound account. First create a [Google Trust Services account](https://cloud.google.com/certificate-manager/docs/public-ca-tutorial#request-key-hmac):
+
+- provide the pre-generated [EAB credentials](https://tools.ietf.org/html/rfc8555#section-7.3.4) using the `ACME_EAB_KID` and `ACME_EAB_HMAC_KEY` environment variables.
+
+These variables can be set on the proxied containers or directly on the **acme-companion** container.

docs/README.md

@@ -16,6 +16,8 @@
 
 [Container configuration](./Container-configuration.md)
 
+[Google Trust Services](./Google-Trust-Services.md)
+
 [Persistent data](./Persistent-data.md)
 
 [Standalone certificates](./Standalone-certificates.md)