docs(readme): clone over https

SSH clones need a registered key even for a public repo, so a fresh
machine got "Permission denied (publickey)". HTTPS works anonymously.
Document switching the push URL to SSH for machines that push.

README.md

@@ -5,10 +5,16 @@ These are my config files for macOS and KDE, managed with GNU Stow. You need `st
 ## Installation
 
 ```sh
-git clone git@ssh.trivk.dev:tri/.dotfiles.git "$HOME/.dotfiles"
+git clone https://git.trivk.dev/tri/.dotfiles.git "$HOME/.dotfiles"
 cd "$HOME/.dotfiles"
 ```
 
+The HTTPS clone works without an account. To push from this machine, add its SSH key on git.trivk.dev and point the push URL at SSH:
+
+```sh
+git remote set-url --push origin git@ssh.trivk.dev:tri/.dotfiles.git
+```
+
 The packages are split into three groups:
 
 - [common/](common/README.md) has Fastfetch, Git, Micro, mpv, npm, Starship and Zsh.