docs: Add authenticated-repos.md

Document options for accessing repositories that require authentication.

docs/authenticated-repos.md

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+# Handling access to authenticated remote repositories
+{: .no_toc }
+
+1. TOC
+{:toc}
+
+
+There is no default concept of an "ostree server"; ostree expects to talk to a generic webserver, so any tool and technique applicable for generic HTTP can also apply to fetching content via OSTree's builtin HTTP client.
+
+## Using mutual TLS
+
+The `tls-client-cert-path` and `tls-client-key-path` expose the underlying HTTP code for [mutual TLS](https://en.wikipedia.org/wiki/Mutual_authentication).
+
+Each device can be provisioned with a secret key which grants it access to the webserver.
+
+## Using basic authentication
+
+The client supports HTTP `basic` authentication, but this has well-known management drawbacks.
+
+## Using cookies
+
+Since [this pull request](https://github.com/ostreedev/ostree/pull/531) ostree supports adding cookies to a remote configuration.  This can be used with e.g. [Amazon CloudFront](https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/private-content-signed-cookies.html).
+
+