docs: Point people to git-branch-stash

Before, git-branch-stash was bundled with git-stack.  We've separated
them, so we need to tell the user where to get it.

Fixes #221

Author: epage

README.md

@@ -95,6 +95,13 @@ Or use rust to install:
 $ cargo install git-stack
 ```
 
+We also recommend installing
+[`git-branch-stash`](https://github.com/gitext-rs/git-branch-stash) for easily
+undoing `git stack` operations:
+```console
+$ cargo install git-branch-stash-cli
+```
+
 ### Uninstall
 
 See the uninstall method for your installer.
@@ -204,6 +211,14 @@ If a commit summary is only `WIP` or is prefixed by:
 
 *This includes the prefixes used by [Gitlab](https://docs.gitlab.com/ee/user/project/merge_requests/drafts.html)*
 
+### What is `git branch-staash`
+
+[`git-branch-stash`](https://github.com/gitext-rs/git-branch-stash) is a
+separate utility that is like `git stash` for instead of your working tree, it
+stashes what commit each of your branches points to.  `git stack` backs up
+using `git branch-stash`s file format to lower the risk of trying things out
+with `git stack`.
+
 ### Why don't you just ...?
 
 Have an idea, we'd love to [hear it](https://github.com/gitext-rs/git-stack/discussions)!

docs/reference.md

@@ -104,7 +104,9 @@ Why not `git push --set-upstream --force-with-lease origin <branch>`?
 
 ### `git branch-stash`
 
-While `git stash` backs up and restores your working tree, `git branch-stash` backs up and restores the state of all of your branches.
+While `git stash` backs up and restores your working tree,
+[`git branch-stash`](https://github.com/gitext-rs/git-branch-stash) backs up
+and restores the state of all of your branches.
 
 `git-stack` implicitly does a `git branch-stash` whenever modifying the tree.