add WSL section to README.md

README.md

@@ -10,8 +10,7 @@ them out. Any command it does not implement itself is passed straight through to
 
 ## Requirements
 
-- A Debian-based host with `bash` and `curl` — a Debian-based WSL instance works as well,
+- A Debian-based host with `bash` and `curl`
-  which lets you back up Windows directories through `/mnt/c`
 - `jq` and `proxmox-backup-client` — both installed by `pbc install`
 - A reachable PBS with a datastore and an API token
 
@@ -239,6 +238,35 @@ cp -a /mnt/restore/etc/nginx /etc/nginx
 umount /mnt/restore
 ```
 
+## Running in WSL
+
+`pbc` also works inside a Debian-based WSL instance, which makes it a way to back up
+Windows directories: the Windows drives show up under `/mnt`, so they can be listed in
+`BACKUP` like any other path.
+
+```bash
+BACKUP=('users.pxar:/mnt/c/Users' 'projects.pxar:/mnt/d/projects')
+```
+
+A few things to keep in mind:
+
+- Access to files under `/mnt` is governed by Windows, not by the user you are inside the
+  WSL instance — `sudo` does not help there. To back up paths your Windows user cannot
+  read, start the WSL instance itself as administrator (run the terminal or `wsl.exe` via
+  *Run as administrator*), then run `pbc` in it.
+- The owner and permission metadata stored in the archive is the one WSL synthesizes for
+  Windows files, not the original Windows ACLs.
+- Cron is not running in a WSL instance by default, so `backup-cron` only fires if you
+  enable it — either by starting `cron` yourself, or by triggering `pbc backup-cron` from
+  the Windows Task Scheduler with `wsl.exe`:
+
+  ```
+  wsl.exe -d Debian -u root /usr/local/bin/pbc backup-cron
+  ```
+
+  For the reason above, such a task has to run with highest privileges to reach files that
+  are not accessible to your Windows user.
+
 ## License
 
 GPLv3 — see [LICENSE](LICENSE).