Update README.md to include venv activate (#1411)

This is just a small readme change to include venv activation.
In case someone just follows the docs as-is, they will end up installing
packages outside of the desired virtual env.

---------

Co-authored-by: Zanie Blue <contact@zanie.dev>

README.md

@@ -53,6 +53,16 @@ To create a virtual environment:
 uv venv  # Create a virtual environment at .venv.
 ```
 
+To activate the virtual environment:
+
+```shell
+# On macOS and Linux.
+source .venv/bin/activate
+
+# On Windows.
+.\.venv\Scripts\activate.ps1
+```
+
 To install a package into the virtual environment:
 
 ```shell