Update documentation to mention the environment variables that scripts can use. (#191)

README.md

@@ -212,7 +212,7 @@ Config properties:
     // This script will only run when Spotify sends a notification containing
     // that exact summary and body
     "example-script": {
-      "exec": "~/.config/swaync/myRickRollScript.sh",
+      "exec": "/path/to/myRickRollScript.sh",
       "app-name": "Spotify"
       "summary": "Never Gonna Give You Up",
       "body": "Rick Astley - Whenever You Need Somebody"

man/swaync.5.scd

@@ -353,3 +353,16 @@ config file to be able to detect config errors
 	}
 }
 ```
+
+	You can also use these environment variables in your script:
+```
+SWAYNC_BODY="Notification body content"
+SWAYNC_DESKTOP_ENTRY="Desktop entry"
+SWAYNC_URGENCY="Notification urgency"
+SWAYNC_TIME="Notification time"
+SWAYNC_APP_NAME="Notification app name"
+SWAYNC_CATEGORY="SwayNC notification category"
+SWAYNC_REPLACES_ID="ID of notification to replace"
+SWAYNC_ID="SwayNC notification ID"
+SWAYNC_SUMMARY="Notification summary"
+```