update documentation to describe router-mode environment variables

Signed-off-by: Phill Kelley <34226495+Paraphraser@users.noreply.github.com>

README.md

@@ -41,6 +41,78 @@ It is the implementation of the local network router [paper](https://zerotier.at
 
 That will start the zero-one, establish connection and build the NAT+router once the `zt` interface is up.
 
+##### Environment variables
+
+The following environment variables are supported:
+
+* `TZ` – timezone support. Example:
+
+	``` yaml
+	TZ=Australia/Sydney
+	```
+
+	Defaults to `Etc/UTC` if omitted.
+
+* `PUID` + `PGID` – user and group IDs for ownership of persistent store. Example:
+
+	``` yaml
+	PUID=1000
+	PGID=1000
+	```
+	
+	If omitted, `PUID` defaults to user ID 999, while `PGID` defaults to group ID 994. These variables are only used to ensure consistent ownership on each launch. They do not affect how the container *runs.* Absent a `user:` directive, the container runs as root and does not downgrade its privileges.
+
+* `ZEROTIER_ONE_LOCAL_PHYS` - controls which physical interfaces participate in network address translation (NAT). Examples:
+
+	- Use only the physical Ethernet interface (this is also the default of the variable is omitted):
+
+		``` yaml
+		ZEROTIER_ONE_LOCAL_PHYS=eth0
+		```
+
+	- If your computer only has WiFi active:
+
+		``` yaml
+		ZEROTIER_ONE_LOCAL_PHYS=wlan0
+		```
+	
+	- If your computer has both Ethernet and WiFi interfaces active and you wish to be able to route through each interface:
+
+		- if using `docker run`:
+
+			``` console
+			--env ZEROTIER_ONE_LOCAL_PHYS="eth0 wlan0"
+			```
+	
+		- if using `docker-compose`:
+
+			``` yaml
+			environment:
+			- ZEROTIER_ONE_LOCAL_PHYS=eth0 wlan0
+			```
+
+* `ZEROTIER_ONE_USE_IPTABLES_NFT` - controls the command the container uses to set up NAT forwarding. Example:
+
+	``` yaml
+	ZEROTIER_ONE_USE_IPTABLES_NFT=true
+	```
+
+	Defaults to `false` if omitted. Try `true` if NAT does not seem to be working.
+	
+* `ZEROTIER_ONE_NETWORK_ID` – auto-join network on first launch. Example:
+
+	``` yaml
+	ZEROTIER_ONE_NETWORK_ID=565758596a6b6c44
+	```
+	
+	This variable is only effective on first launch. There is no default if it is omitted. It is the equivalent of running the following command after the container first starts:
+	
+	```
+	$ docker exec zerotier zerotier-cli join 565758596a6b6c44
+	```
+
+	It does not matter whether you use this environment variable or the `join` command, you still need to authorize the computer in ZeroTier Central.
+
 #### Source
 
 https://github.com/zyclonite/zerotier-docker