Add Tr-compiler write-up and doc-comment

This write-up is aimed to reason about how the private-compilation (as defined in doc) is also cost-efficient.

Author: SarcasticNastik

doc/Tr Compiler.pdf

@@ -248,6 +248,9 @@ impl<Pk: MiniscriptKey> Policy<Pk> {
     /// `[pk(A),pk(B),and(or(pk(C),pk(D)),pk(E)))]`. Each policy in the vector is compiled into
     /// the respective miniscripts. A Huffman Tree is created from this vector which optimizes over
     /// the probabilitity of satisfaction for the respective branch in the TapTree.
+    ///
+    /// Refer to [doc/Tr compiler.pdf] in the root of the repository to understand why such
+    /// compilation is also *cost-efficient*.
     // TODO: We might require other compile errors for Taproot.
     #[cfg(feature = "compiler")]
     pub fn compile_tr(&self, unspendable_key: Option<Pk>) -> Result<Descriptor<Pk>, Error> {