docs: adds transform configuration documentation to README

References #1

Author: fcoury

README.md

@@ -41,6 +41,43 @@ npm install --global json-log-viewer
 jv application.log.2017-01-01 --sort -timestamp
 ```
 
+### Configuration
+
+The default expected log format include fields `timestamp`, `level` and `message`. If the log file you're trying to parse doesn't include those fields, you can create a config file on your HOME path called `.json-log-viewer`.
+
+For example, if your log lines look like this:
+
+```json
+{
+  "message":
+    "Matched route \"**_heartbeat_check\" (parameters: \"_controller\": \"**\\Controller\\**Controller::heartbeatCheckAction\", \"_route\": \"**_heartbeat_check\")",
+  "context": [],
+  "level": 200,
+  "level_name": "INFO",
+  "channel": "request",
+  "datetime": {
+    "date": "2017-12-06 09:23:42.253060",
+    "timezone_type": 3,
+    "timezone": "Europe/Berlin"
+  },
+  "extra": []
+}
+```
+
+You can create a mapping configuration like this:
+
+```ini
+[transform]
+level=level_name
+timestamp=datetime.date
+message=message
+extra=$
+```
+
+This way the messages will properly be displayed. The `$` has a special meaning: it tells the the old object should be included on the `extra` key on the resulting JSON. The result will look like this:
+
+![transform](transform.png)
+
 ## Screenshots
 
 __Details view__