Add immediate safety warning to README (#12)

* Add immediate safety warning to README

- Remove the production-use claim from Project Status.
- Mark the project experimental until the P0 data-safety checklist
  passes, with a prominent warning banner up top.
- Add a Limitations section: warn against modifying hardlinked
  snapshots in place, document the lack of a restore command, and
  note special-entry exclusion and symlink-preservation behavior.
- Check off the corresponding IMPROVEMENTS.md acceptance criteria.

* Qualify special-entry exclusion limitation to the Python implementation

--external-rsync uses rsync's --archive flag, which copies/recreates
FIFOs, sockets, and device nodes instead of excluding them - the
exclusion behavior only applies to the default Python implementation.

* Qualify device-node recreation reliability under --external-rsync

rsync -a implies -D (devices/specials), but creating device nodes
typically requires root on the destination - without it rsync may
skip or fail on them rather than reliably recreating them.

---------

Co-authored-by: Maxim Shulga <maxim.shulga@alludo.com>

IMPROVEMENTS.md

@@ -120,10 +120,10 @@ Acceptance criteria:
 
 ### Add an immediate safety warning
 
-- [ ] Remove the current production-use claim from `README.md`.
-- [ ] State that the software is experimental until the P0 release gate passes.
-- [ ] Warn users not to modify hardlinked snapshots in place.
-- [ ] Document current recovery and metadata limitations honestly.
+- [x] Remove the current production-use claim from `README.md`.
+- [x] State that the software is experimental until the P0 release gate passes.
+- [x] Warn users not to modify hardlinked snapshots in place.
+- [x] Document current recovery and metadata limitations honestly.
 
 ## P1 - Recovery and release readiness
 

README.md

@@ -1,5 +1,10 @@
 # cura-te-ipsum
 
+> **⚠️ Experimental — not yet safe for production use.** This project is
+> still working through its data-safety checklist (see
+> [IMPROVEMENTS.md](IMPROVEMENTS.md)). Do not rely on it as your only
+> backup, and see [Limitations](#limitations) before using it.
+
 **cura-te-ipsum** is a space-efficient incremental backup utility for Linux and macOS that uses hardlinks to minimize storage usage while maintaining complete directory snapshots.
 
 Similar to Time Machine or rsnapshot, cura-te-ipsum creates backups that appear as complete directory trees but intelligently share unchanged files between snapshots, dramatically reducing storage requirements.
@@ -127,6 +132,28 @@ backups/
   .backups_lock                 # lock file (only during backup)
 ```
 
+## Limitations
+
+- **Never modify files inside a backup snapshot in place.** Unchanged
+  files are hardlinked between snapshots to save space, so editing or
+  truncating a file in one snapshot changes that same file's content
+  in every other snapshot sharing its inode. Treat every snapshot
+  directory as read-only; copy files out before editing them.
+- **No dedicated restore command.** Recovering data means manually
+  copying files out of a snapshot directory - there is no restore
+  workflow with dry-run, partial restore, or overwrite-policy support
+  yet.
+- **Special filesystem entries (FIFOs, sockets, device nodes) are
+  excluded, not backed up** - but only with the default Python
+  implementation, which records them in the snapshot manifest as
+  exclusions rather than copying them. With `--external-rsync`,
+  `rsync`'s `--archive` flag attempts to recreate these entries
+  instead, though device nodes typically require root privileges on
+  the destination and may be skipped or fail without them.
+- **Symlinks are preserved as symlinks**, including ones pointing
+  outside the source tree - their targets are never followed or
+  copied.
+
 ## Development
 
 ### Running Tests
@@ -146,6 +173,8 @@ Maks Snegov (<snegov@spqr.link>)
 
 ## Project Status
 
-Development Status: Pre-Alpha
+Development Status: Pre-Alpha, experimental
 
-This project is actively maintained and used in production for personal backups, but the API and configuration options may change in future releases.
+This project is under active development and has not yet completed
+its data-safety checklist (see [IMPROVEMENTS.md](IMPROVEMENTS.md)).
+The API and configuration options may change in future releases.