Add deprecation notice to cranelift-faerie README and FaerieBuilder.

cranelift/faerie/src/backend.rs

@@ -34,6 +34,12 @@ impl FaerieBuilder {
     /// enum to symbols. LibCalls are inserted in the IR as part of the legalization for certain
     /// floating point instructions, and for stack probes. If you don't know what to use for this
     /// argument, use `cranelift_module::default_libcall_names()`.
+    #[deprecated(
+        since = "0.65.0",
+        note = "the Cranelift developer team intends to stop maintaining the `cranelift-faerie`
+        crate and remove it from the `wasmtime` git repository on or after August 3, 2020. We
+        recommend users use its successor, the `cranelift-object` crate."
+    )]
     pub fn new(
         isa: Box<dyn TargetIsa>,
         name: String,