externals: Document subtrees

externals/README.md

@@ -0,0 +1,19 @@
+This repository uses subtrees to manage some of its externals.
+
+## Initial setup
+
+```
+git remote add externals-fmt https://github.com/fmtlib/fmt.git
+git remote add externals-xbyak https://github.com/herumi/xbyak.git
+```
+
+## Updating
+
+Change `<ref>` to refer to the appropriate git reference.
+
+```
+git fetch externals-fmt
+git fetch externals-xbyak
+git subtree pull --squash --prefix=externals/fmt externals-fmt <ref>
+git subtree pull --squash --prefix=externals/xbyak externals-xbyak <ref>
+```