Add AWS auth and secret docs

Add docs to README explaining how to enable and use aws auth and aws
client secrets.

policies/aws_wireguard.hcl

@@ -0,0 +1,7 @@
+path "secret/data/wireguard/aws/*" {
+    capabilities = ["read", "list"]
+}
+
+path "secret/metadata/wireguard/aws/*" {
+    capabilities = ["read"]
+}

policies/ducoterra.hcl

@@ -29,3 +29,7 @@ path "ssh-host-signer/sign/*" {
 path "ssh-host-signer/roles/*" {
     capabilities = ["create", "read", "update", "delete", "list"]
 }
+
+path "aws/*" {
+    capabilities = ["create", "read", "update", "delete", "list"]
+}