Document firehol_level1 RFC1918 gotcha in blocklist defaults

The default [defense.blocklist] uses firehol_level1.netset, which
includes bogon networks and therefore all RFC1918 ranges. Clients
connecting from a LAN address (e.g. a phone on the home Wi-Fi when
mtg runs at home) are silently rejected with "ip was blacklisted"
and routed to the fronting domain. This is a recurring source of
confusion (see issue #466 for the latest example).

Add a warning next to the urls list in example.config.toml and a
Troubleshooting section in README.md covering the symptom, the
cause, and three resolution paths (disable blocklist, swap for a
narrower list, or use hairpin NAT).

Docs only, no code changes.

README.md

    you can enable `drs` setting.
 9. **If you are not sure, touch nothing!**
 
+## Troubleshooting
+
+### `ip was blacklisted` for clients on the same LAN
+
+If you run mtg at home and a client on the same LAN (for example, your
+phone on the home Wi-Fi) cannot connect, check the proxy logs for a
+message like:
+
+```json
+{"level":"info","ip":"10.0.1.1","logger":"proxy","message":"ip was blacklisted"}
+```
+
+The reason is that the default blocklist (`firehol_level1.netset`)
+includes bogon networks, which covers all RFC1918 ranges
+(`10.0.0.0/8`, `172.16.0.0/12`, `192.168.0.0/16`). Any client
+connecting from such an address is rejected by the blocklist and
+silently routed to the fronting domain.
+
+There are three ways to resolve it:
+
+1. Disable the blocklist entirely in `config.toml`:
+
+   ```toml
+   [defense.blocklist]
+   enabled = false
+   ```
+
+   Simplest option if the proxy is used only by you and people you trust.
+
+2. Keep the blocklist but swap `firehol_level1` for a narrower list that
+   does not include bogons, for example `firehol_abusers_1d`:
+
+   ```toml
+   [defense.blocklist]
+   enabled = true
+   urls = ["https://iplists.firehol.org/files/firehol_abusers_1d.netset"]
+   ```
+
+3. Connect to the proxy through a public IP or domain name with hairpin
+   NAT (`MASQUERADE`) on your router. mtg will then see the client with
+   its public address and the blocklist will not match. This is more
+   work to set up but preserves full blocklist protection.
+
 ## Metrics
 
 Out of the box, mtg works with

example.config.toml

 # A list of URLs in FireHOL format (https://iplists.firehol.org/)
 # You can provider links here (starts with https:// or http://) or
 # path to a local file, but in this case it should be absolute.
+#
+# NOTE: the default list below (firehol_level1.netset) includes bogon
+# networks, and therefore RFC1918 ranges as well (10.0.0.0/8,
+# 172.16.0.0/12, 192.168.0.0/16). If you run mtg on a home/LAN network
+# and connect from a client on the same LAN, that client will be
+# rejected with "ip was blacklisted" and silently routed to the fronting
+# domain. If you see this, you can either disable this section
+# (enabled = false), replace firehol_level1 with a narrower list that
+# does not include bogons (e.g. firehol_abusers_1d), or connect via
+# a public IP/domain with hairpin NAT on your router. See README for
+# details.
 urls = [
     "https://iplists.firehol.org/files/firehol_level1.netset",
     # "/local.file"