docs: add README with install, configuration and migration

Written for someone who has never seen the pre-split extension: what the plugin adds to Nemo, what it costs, every config key, and the exact steps to move over from the single file version or go back.

README.md

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+# nemo-folder-tools
+
+"Open Terminal Here" and "Open in Editor" in the right click menu of
+[Nemo](https://github.com/linuxmint/nemo), the Cinnamon file manager.
+
+Two entries, on any local folder, on the folder icon and on the empty
+background of an open folder:
+
+| Entry | What it does |
+| --- | --- |
+| Open Terminal Here | Opens your terminal emulator with the folder as its working directory |
+| Open in Editor | Opens the folder in the editor you configured, `code` by default |
+
+Nothing here is git specific. It is the part of a larger repository plugin that
+turned out to be useful everywhere, so it is packaged on its own.
+
+An entry is only added when it can actually work: no terminal on the system
+means no Open Terminal Here, an editor command that is not installed means no
+Open in Editor, rather than a menu entry that silently does nothing.
+
+## Requirements
+
+- [nemo-repo-core](https://github.com/dualfroz/nemo-repo-core) 1.x, for the
+  shared config file and the launcher helpers
+- Nemo with `nemo-python`
+- A terminal emulator. `gnome-terminal`, `tilix`, `konsole`, `xfce4-terminal`,
+  `mate-terminal`, `kitty`, `alacritty` and `xterm` are detected automatically;
+  anything else can be named in the config.
+
+## Install
+
+```bash
+git clone https://github.com/dualfroz/nemo-repo-core
+cd nemo-repo-core && ./install.sh && cd ..
+
+git clone https://github.com/dualfroz/nemo-folder-tools
+cd nemo-folder-tools && ./install.sh
+
+nemo -q
+```
+
+`install.sh` links the plugin into `~/.local/share/nemo-python/extensions` and
+checks that it imports. Pass `--copy` to copy instead of link. `./uninstall.sh`
+removes it. Nothing is written outside your home directory and no root is
+required.
+
+## Configuration
+
+Shared file, read live, no restart needed:
+
+```
+~/.config/nemo-repo-core.json
+```
+
+```json
+{
+  "editor_command": "code",
+  "terminal_command": null,
+  "actions": {
+    "open_terminal": true,
+    "open_editor": true
+  }
+}
+```
+
+| Key | Default | Meaning |
+| --- | --- | --- |
+| `actions.open_terminal` | `true` | Show the Open Terminal Here entry |
+| `actions.open_editor` | `true` | Show the Open in Editor entry |
+| `editor_command` | `"code"` | Command to open the folder with. May include arguments, for example `"code -n"` |
+| `terminal_command` | `null` | Terminal to use; `null` auto detects |
+
+`editor_command` and `terminal_command` belong to the core and are shared with
+the other plugins in the family, so setting your editor once covers all of
+them.
+
+Some editors people set here: `code`, `codium`, `subl`, `zed`, `nvim` inside a
+terminal wrapper, or `xdg-open` to hand the folder to whatever the desktop
+considers the default.
+
+## Migrating from the single file version
+
+These two entries used to live in `nemo_repo_emblems.py` together with git
+emblems, columns, a properties tab and git menu actions. They work on every
+folder and have nothing to do with git, so they are their own plugin now. The
+git actions moved to
+[nemo-repo-git-menu](https://github.com/dualfroz/nemo-repo-git-menu).
+
+One change worth knowing: the Open in Editor entry used to appear only when
+Visual Studio Code specifically was installed, no matter what `editor_command`
+said. It now checks the command you actually configured.
+
+Your config carries over automatically, and the old file is not modified.
+Remove the old extension so the entries are not listed twice:
+
+```bash
+rm ~/.local/share/nemo-python/extensions/nemo_repo_emblems.py
+nemo -q
+```
+
+## Tested on
+
+Linux Mint 22.2 (Zara), Cinnamon 6.4.14, Nemo 6.4.5, python-nemo 6.4.0,
+Python 3.12.3, with `gnome-terminal` and `code`.
+
+## License
+
+MIT. See `LICENSE`.

config.example.json

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+{
+  "editor_command": "code",
+  "terminal_command": null,
+  "actions": {
+    "open_terminal": true,
+    "open_editor": true
+  }
+}