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-repo-properties
+
+A "Git" tab in the folder Properties dialog of
+[Nemo](https://github.com/linuxmint/nemo), the Cinnamon file manager.
+
+Right click a repository, choose Properties, and next to Basic and Permissions
+there is a Git tab:
+
+```
+Repository       /home/you/code/project
+Branch           feature/parser
+Upstream         origin/feature/parser
+Ahead / behind   2 ahead, 0 behind
+Staged changes   1
+Unstaged changes 3
+Untracked files  0
+Summary          2 ahead, 4 changed
+Last commit      a1b2c3d  fix the token boundary case
+                 by You, 2 hours ago
+```
+
+Every value is selectable, so a branch name or a commit hash can be copied
+straight out of the dialog.
+
+The tab only appears on repository roots. On any other folder the Properties
+dialog is unchanged.
+
+## Requirements
+
+- [nemo-repo-core](https://github.com/dualfroz/nemo-repo-core) 1.x
+- Nemo with `nemo-python`, `git` in `PATH`
+
+## 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-repo-properties
+cd nemo-repo-properties && ./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
+
+None of its own. It reads the shared file,
+`~/.config/nemo-repo-core.json`, only for core settings such as `log_level`.
+
+Opening Properties is an explicit request for the current state, so this plugin
+deliberately bypasses the shared cache and asks git directly every time. The
+`cache_ttl_seconds` setting does not affect it.
+
+## Tested on
+
+Linux Mint 22.2 (Zara), Cinnamon 6.4.14, Nemo 6.4.5, python-nemo 6.4.0,
+Python 3.12.3.
+
+## Migrating from the single file version
+
+This tab used to be part of `nemo_repo_emblems.py`, one file that also provided
+emblems, list view columns and a context menu. It is now a separate plugin on a
+shared core, so it can be installed on its own.
+
+Your config carries over automatically, and the old config file is not
+modified. Remove the old extension so the tab does not appear twice:
+
+```bash
+rm ~/.local/share/nemo-python/extensions/nemo_repo_emblems.py
+nemo -q
+```
+
+## License
+
+MIT. See `LICENSE`.