docs: add workaround for fish shell (cashapp#395)

This updates the documentation to support `fish` shell using a
workaround via `direnv`.

docs/docs/faq/index.md

@@ -6,8 +6,50 @@ hide:
 
 ## Which Shells Does Hermit work with?
 
-Hermit currently works with ZSH and BASH, but we would welcome 
-[contributions](https://github.com/cashapp/hermit/pulls) to support other shells.
+Hermit currently works with `bash` and `zsh` natively. `fish` is available via
+a [workaround](#fish-support-via-direnv). But we would welcome
+[contributions](https://github.com/cashapp/hermit/pulls) to support other
+shells.
+
+### `fish` support via `direnv`
+
+Hermit does not currently support `fish` directly, but you can use `direnv` to
+achieve similar functionality.
+
+1. Install `direnv` via your package manager.
+2. Create `$HOME/.direnvrc` with the following [content](#direnvrc).
+3. Create a `project_dir/.envrc` with the following [content](#envrc).
+4. Use `direnv allow` to activate the environment (usually only needed on
+ modification of `.envrc`)
+
+
+#### `~/.direnvrc`
+
+```bash
+# shellcheck shell=bash
+
+# derived from https://github.com/direnv/direnv/wiki/Python#virtualenvwrapper
+layout_hermit() {
+ local activate_hermit
+ activate_hermit="$(\
+ find . \
+ -type f \
+ -name 'activate-hermit' \
+ -exec test -x {} \; \
+ -print
+ )"
+ if [[ -n "$activate_hermit" ]];then
+ # shellcheck source=/dev/null
+ source "$activate_hermit"
+ fi
+}
+```
+
+#### `.envrc`
+
+```bash
+layout hermit
+```
 
 ### powerlevel10k support
 If you would like powerlevel10k to support hermit, all that is needed is to add the following to your `~/.p10k.zsh`