Add note about config

Former-commit-id: b6e9aae692

README.md

@@ -48,6 +48,7 @@ All CLI arguments can also be set via environment variables:
 -   `PING_INTERVAL`: Equivalent to `--ping-interval`
 -   `PING_TIMEOUT`: Equivalent to `--ping-timeout`
 -   `HOLEPUNCH`: Set to "true" to enable hole punching (equivalent to `--holepunch`)
+-   `CONFIG_FILE`: Set to the location of a JSON file to load secret values
 
 Example:
 
@@ -58,6 +59,28 @@ olm \
 --endpoint https://example.com
 ```
 
+## Loading secrets from files
+
+You can use `CONFIG_FILE` to define a location of a config file to store the credentials between runs. 
+
+```
+$ cat ~/.config/olm-client/config.json
+{
+  "id": "spmzu8rbpzj1qq6",
+  "secret": "f6v61mjutwme2kkydbw3fjo227zl60a2tsf5psw9r25hgae3",
+  "endpoint": "https://pangolin.fossorial.io",
+  "tlsClientCert": ""
+}
+```
+
+This file is also written to when newt first starts up. So you do not need to run every time with --id and secret if you have run it once! 
+
+Default locations: 
+
+- **macOS**: `~/Library/Application Support/olm-client/config.json`
+- **Windows**: `%PROGRAMDATA%\olm\olm-client\config.json`
+- **Linux/Others**: `~/.config/olm-client/config.json`
+
 ## Hole Punching
 
 In the default mode, olm "relays" traffic through Gerbil in the cloud to get down to newt. This is a little more reliable. Support for NAT hole punching is also EXPERIMENTAL right now using the `--holepunch` flag. This will attempt to orchestrate a NAT hole punch between the two sites so that traffic flows directly. This will save data costs and speed. If it fails it should fall back to relaying.