docs: add note about hooking asdf into your shell

docs/developer-setup.md

@@ -33,6 +33,24 @@ brew install asdf
 Linux users should follow the instructions for their package manager in [the ASDF documentation](https://asdf-vm.com/guide/getting-started).
+#### Step 2.5: Hook `asdf` into your shell
+
+Once you've installed `asdf`, add the following line to your shell. Remember to restart your terminal for the change to take affect.
+
+### Macos with Homebrew
+
+```shell
+. $(brew --prefix asdf)/asdf.sh
+```
+
+### Linux
+
+```shell
+. <path to asdf install>/asdf.sh
+```
+
+Did you remember to restart your terminal? Good.
+
 ### Step 2: Install development dependencies
 Install all the plugins by running: