Add upgrade documentation for the build refactor

Author: filmor

UPGRADE.md

@@ -2,6 +2,23 @@
 
 This document is intended to simplify upgrading to newer versions by extending the changelog.
 
+## 0.36 -> 0.37
+
+`rustler_mix` will now require a workspace with the same root as the Elixir
+package with all relevant crates included as workspace members. If the package
+previously had a crate at `native/my_crate`, a `Cargo.toml` with the following
+content should be added at the root:
+```toml
+[workspace]
+resolver = "3"
+members = ["native/my_crate"]
+```
+
+The `/target` directory should also be added to the root's `.gitignore` file.
+
+More information on workspaces can be found in the Rust documentation at
+https://doc.rust-lang.org/book/ch14-03-cargo-workspaces.html.
+
 ## 0.34 -> 0.35
 
 `rustler_sys` as a standalone library has been replaced by an embedded