docs: install subtree

.README.md

@@ -44,6 +44,16 @@ dotfiles checkout
 lazygit --git-dir=$HOME/.dotfiles --work-tree=$HOME
 ```
 
+# Install remotes as subtree
+
+- https://www.atlassian.com/git/tutorials/git-subtree
+- Create an alias for remote repository
+  - `dotfiles remote add <remote-name> <URL>`
+- Import remote as a single commit (squashed)
+  - `dotfiles subtree add --prefix <target-folder> <remote-name> <branch> --squash
+  - Note 1: Must be called from $HOME
+  - Note 2: Do not start <target-folder> with $HOME or ~
+
 # Install recent versions of applications
 
 ## Generic Tips