Add paragraph about reusing ACME accounts

docs/production/index.rst

@@ -511,3 +511,45 @@ The following must be added to the config file to activate the pinning (the pinn
     KOqkqm57TH2H3eDJAkSnh6/DNFu0Qg==
     -----END CERTIFICATE-----
     """
+
+
+LetsEncrypt: Using a pre-existing ACME account
+-----------------------------------------------
+
+Let's Encrypt allows reusing an existing ACME account, to create and especially revoke certificates. The current
+implementation in the acme plugin, only allows for a single account for all ACME authorities, which might be an issue,
+when you try to use Let's Encrypt together with another certificate authority that uses the ACME protocol.
+
+To use an existing account, you need to configure the `ACME_PRIVATE_KEY` and `ACME_REGR` variables in the lemur
+configuration.
+
+`ACME_PRIVATE_KEY` needs to be in the JWK format::
+
+    {
+        "kty": "RSA",
+        "n": "yr1qBwHizA7ME_iV32bY10ILp.....",
+        "e": "AQAB",
+        "d": "llBlYhil3I.....",
+        "p": "-5LW2Lewogo.........",
+        "q": "zk6dHqHfHksd.........",
+        "dp": "qfe9fFIu3mu.......",
+        "dq": "cXFO-loeOyU.......",
+        "qi": "AfK1sh0_8sLTb..........."
+    }
+
+
+Using `python-jwt` converting a existing private key in PEM format is quite easy::
+
+    import python_jwt as jwt, jwcrypto.jwk as jwk
+
+    priv_key = jwk.JWK.from_pem(b"""-----BEGIN RSA PRIVATE KEY-----
+    ...
+    -----END RSA PRIVATE KEY-----""")
+
+    print(priv_key.export())
+
+`ACME_REGR` needs to be a valid JSON with a `body` and a `uri` attribute, similar to this::
+
+    {"body": {}, "uri": "https://acme-staging-v02.api.letsencrypt.org/acme/acct/<ACCOUNT_NUMBER>"}
+
+The uri can be retrieved from the ACME create account endpoint, when trying to create a new account, using the existing key.