Cross-link documentation for adding a new target

Both the target tier policy and the rustc-dev-guide has documentation on
this, let's make sure people see both.

src/doc/rustc-dev-guide/src/building/new-target.md

@@ -4,8 +4,13 @@ These are a set of steps to add support for a new target. There are
 numerous end states and paths to get there, so not all sections may be
 relevant to your desired goal.
 
+See also the associated documentation in the
+[target tier policy][target_tier_policy_add].
+
 <!-- toc -->
 
+[target_tier_policy_add]: https://doc.rust-lang.org/rustc/target-tier-policy.html#adding-a-new-target
+
 ## Specifying a new LLVM
 
 For very new targets, you may need to use a different fork of LLVM