Add usage to README

README.md

@@ -6,3 +6,36 @@ Generate bird 2.0 roa entries from the dn42 registry.
 
 1. Install `cargo-deb`: `cargo install cargo-deb`
 2. Build dpkg: `cargo deb`
+
+## Usage
+
+Add
+```bird
+
+roa4 table dn42_roa4;
+roa6 table dn42_roa6;
+
+protocol static {
+    roa4 {
+        table dn42_roa4;
+    };
+
+    roa6 {
+        table dn42_roa6;
+    };
+
+    include "/var/lib/bird/dn42-roa46.conf";
+};
+```
+to your bird config.
+You can now use
+```
+if (roa_check(dn42_roa4, net, bgp_path.last) != ROA_VALID) then {
+    reject;
+}
+
+if (roa_check(dn42_roa6, net, bgp_path.last) != ROA_VALID) then {
+    reject;
+}
+```
+to reject any routes without a matching and valid roa entry.