Add docs for remote access

docs/remote-cli.md

@@ -0,0 +1,85 @@
+# Controlling `headscale` with remote CLI
+
+## Prerequisit
+
+- A workstation to run `headscale` (could be Linux, macOS, other supported platforms)
+- A `headscale` server (version `0.13.0` or newer)
+- Access to create API keys (local access to the `headscale` server)
+- `headscale` _must_ be served over TLS/HTTPS
+  - Remote access does _not_ support unencrypted traffic.
+
+## Goal
+
+This documentation has the goal of showing a user how-to set control a `headscale` instance
+from a remote machine with the `headscale` command line binary.
+
+## Create an API key 
+
+We need to create an API key to authenticate our remote `headscale` when using it from our workstation.
+
+To create a API key, log into your `headscale` server and generate a key:
+
+```shell
+headscale apikeys create --expiration 90d
+```
+
+Copy the output of the command and save it for later. Please not that you can not retrieve a key again, 
+if the key is lost, expire the old one, and create a new key.
+
+To list the keys currently assosicated with the server:
+
+```shell
+headscale apikeys list
+```
+
+and to expire a key:
+
+```shell
+headscale apikeys expire --prefix "<PREFIX>"
+```
+
+
+## Download and configure `headscale`
+
+1. Download the latest [`headscale` binary from GitHub's release page](https://github.com/juanfont/headscale/releases):
+
+2. Put the binary somewhere in your `PATH`, e.g. `/usr/local/bin/headcale`
+
+3. Make `headscale` executable:
+
+```shell
+chmod +x /usr/local/bin/headscale
+```
+
+4. Configure the CLI through Environment Variables
+
+```shell
+export HEADSCALE_CLI_ADDRESS="<HEADSCALE ADDRESS>"
+export HEADSCALE_CLI_API_KEY="<API KEY FROM PREVIOUS STAGE>"
+```
+
+This will tell the `headscale` binary to connect to a remote instance, instead of looking
+for a local instance (which is what it does on the server). 
+
+The API key is needed to make sure that your are allowed to access the server. The key is _not_
+needed when running directly on the server, as the connection is local.
+
+5. Test the connection
+
+Let us run the headscale command to verify that we can connect by listing our nodes:
+
+```shell
+headscale nodes list
+```
+
+You should now be able to see a list of your nodes from your workstation, and you can
+now control the `headscale` server from your workstation.
+
+## Troubleshooting
+
+Checklist:
+
+- Make sure you have the _same_ `headscale` version on your server and workstation
+- Make sure you use version `0.13.0` or newer.
+- Verify that your TLS certificate is valid 
+  - If it is not valid, set the environment variable `HEADSCALE_CLI_INSECURE=true` to allow insecure certs.