README: add Upgrading section (pull, rake update, install.sh)

README.md

@@ -31,6 +31,51 @@ The installer:
 
 Requires `git` and `rake` (ruby).
 
+## Upgrading
+
+Two things can be refreshed independently.
+
+### Pull new drunkendotfiles changes
+
+```sh
+cd ~/.yadr && git pull
+```
+
+That gets you any new config changes, tmux tweaks, helper scripts, etc.
+pushed to this repo. Vim plugins and prezto modules are *not* touched — see
+below.
+
+### Refresh YADR plugins
+
+```sh
+cd ~/.yadr && rake update
+```
+
+`rake update` walks through YADR's plugin managers (Vim-plug for vim, the
+Prezto submodule for zsh) and pulls down the latest versions of each.
+Run this every so often, or after pulling drunkendotfiles changes that
+modify the plugin list (`vim/.vundles`, `vim/after/.vundles.after`, or
+the `modules` list in `zsh/prezto-override/zpreztorc`).
+
+### Redeploy personal dotfiles
+
+If a `git pull` touched personal dotfiles at the repo root (e.g. a new
+`.tmux.conf` or a new `.local/bin/` helper) and you want those changes
+deployed to `$HOME`:
+
+```sh
+~/.yadr/install.sh
+```
+
+It's idempotent — any files in `$HOME` that would be overwritten get
+moved to `~/.drunkendotfiles.bak.<timestamp>/` first.
+
+### Full refresh in one shot
+
+```sh
+cd ~/.yadr && git pull && rake update && ./install.sh
+```
+
 ## What's included
 
 On top of stock YADR: