docs: add mise shell examples and document read_write auto-creation

- Add Fish and Zsh shell activation examples to CONTRIBUTING.md
- Document that read_write directories are automatically created in sandbox.md

CONTRIBUTING.md

@@ -11,6 +11,18 @@ curl https://mise.run | sh
 
 After installing `mise` be sure to activate the environment by running `mise activate` or [add it to your shell](https://mise.jdx.dev/getting-started.html).
 
+Shell instllation examples:
+
+Fish:
+```bash
+echo '~/.local/bin/mise activate fish | source' >> ~/.config/fish/config.fish
+```
+
+Zsh (Mac OS Default):
+```bash
+echo 'eval "$(~/.local/bin/mise activate zsh)"' >> ~/.zshrc
+```
+
 Project uses Rust 1.88+ and Python 3.12+.
 
 ## Getting started
@@ -33,7 +45,6 @@ nav --help
 
 # Run the sandbox
 mise run sandbox
-
 ```
 
 ## Project Structure

architecture/sandbox.md

@@ -23,7 +23,7 @@ Sandboxing is driven by a required policy configuration. There are two ways to p
 
 The policy schema includes:
 
-- `filesystem`: read-only and read-write allow lists, plus optional inclusion of the workdir.
+- `filesystem`: read-only and read-write allow lists, plus optional inclusion of the workdir. Any directories under `read_write` will automatically be created if they do not exist.
 - `network`: mode (`allow`, `block`, `proxy`) and optional proxy configuration.
 - `landlock`: compatibility behavior (`best_effort` or `hard_requirement`).
 - `process`: optional `run_as_user`/`run_as_group` to drop privileges for the child process.