README.md: include sample builder configuration

To highlight how the builder plugin is configured and where to put
the configuration files.

README.md

@@ -11,6 +11,32 @@ plugins:
    command line client. This will then use the new XMLRPC API to request a
    new compose.
 
+## Configuration
+
+The builder plugin needs to be configured via a `builder.conf` file that
+can be located in either `/usr/share/koji-osbuild` or `/etc/koji-osbuild`.
+
+```ini
+[composer]
+# The host, port and transport (https vs http) of osbuild composer
+# NB: The 'https' transport is required for SSL/TLS authorization
+server = https://composer.osbuild.org
+
+# Authorization via client side certificates: can be either a pair of
+# certificate and key files separated by comma or a file combining both.
+ssl_cert = /share/worker-crt.pem, /share/worker-key.pem
+
+# Verification of the server side: either a boolean (True / False) to
+# enable or disable verification, or a path to a CA_BUNDLE file or a
+# directory containing certificates of trusted CAs.
+ssl_verify = /share/worker-ca.pem
+
+[koji]
+# The URL to the koji hub XML-RPC endpoint
+server = https://koji.fedoraproject.org/kojihub
+```
+
+
 ## Development
 
 See [`HACKING.md`](HACKING.md) for how to develop and test this project.