Add a policy example

Add a new policy example demonstrating the bdk's policy structure
and how to derive it for any descriptor without creating a bdk wallet.

Cargo.toml

@@ -113,6 +113,10 @@ name = "miniscriptc"
 path = "examples/compiler.rs"
 required-features = ["compiler"]
 
+[[example]]
+name = "policy"
+path = "examples/policy.rs"
+
 [[example]]
 name = "rpcwallet"
 path = "examples/rpcwallet.rs"