README: /etc/pve (pmxcfs) does not support chmod

pmxcfs ignores chmod, so scripts in /etc/pve cannot be made
executable. Document running them via python3 explicitly, and add
a /usr/local/sbin install variant for those who prefer an
executable script per node.

README.md

@@ -15,11 +15,25 @@ installs needed. Requires `pvesh` (from `pve-manager`) and `dig`
 ## Installation
 
 Copy the script and the config to a Proxmox host (or any host with
-`pvesh` access to the cluster), e.g. to `/etc/pve`:
+`pvesh` access to the cluster).
+
+Note: `/etc/pve` is Proxmox's cluster filesystem (pmxcfs). It syncs
+its files to every node, but does not support `chmod` — scripts
+there cannot be made executable. Two options:
+
+**a) In `/etc/pve`** — cluster-wide, run via `python3`:
 
 ```
 cp manage_ipset.py manage_ipset.conf /etc/pve/
-chmod +x /etc/pve/manage_ipset.py
+python3 /etc/pve/manage_ipset.py
+```
+
+**b) Local and executable** — e.g. `/usr/local/sbin`, per node:
+
+```
+install -m 755 manage_ipset.py /usr/local/sbin/
+install -m 644 manage_ipset.conf /etc/
+/usr/local/sbin/manage_ipset.py -c /etc/manage_ipset.conf
 ```
 
 `manage_ipset.conf` is looked up next to the script unless `-c` is
@@ -29,21 +43,24 @@ given.
 
 ```
 # sync every IPSet from the config
-./manage_ipset.py
+python3 manage_ipset.py
 
 # use a different config file
-./manage_ipset.py -c /path/to/manage_ipset.conf
+python3 manage_ipset.py -c /path/to/manage_ipset.conf
 
 # sync a single IPSet directly
-./manage_ipset.py github github.com github.io
-./manage_ipset.py docker-io registry-1.docker.io --round 30
+python3 manage_ipset.py github github.com github.io
+python3 manage_ipset.py docker-io registry-1.docker.io --round 30
 ```
 
 ### Cron
 
 ```
-# every hour
-0 * * * * /etc/pve/manage_ipset.py
+# every hour, /etc/pve install
+0 * * * * python3 /etc/pve/manage_ipset.py
+
+# local install
+0 * * * * /usr/local/sbin/manage_ipset.py -c /etc/manage_ipset.conf
 ```
 
 ## Config (`manage_ipset.conf`)