feat(#72): add helpful debug info in docs

README.md

@@ -297,6 +297,21 @@ Enable libvips traces (note that a lot of data will be written in stdout):
 VIPS_TRACE=1 ./app 
 ```
 
+You can also dump a core on failure, as [John Cuppit](https://github.com/jcupitt) said: 
+```c
+g_log_set_always_fatal(
+                G_LOG_FLAG_RECURSION |
+                G_LOG_FLAG_FATAL |
+                G_LOG_LEVEL_ERROR |
+                G_LOG_LEVEL_CRITICAL |
+                G_LOG_LEVEL_WARNING );
+```
+
+Or set the G_DEBUG environment variable:
+```
+export G_DEBUG=fatal-warnings,fatal-criticals
+```
+
 ## API 
 
 See [godoc reference](https://godoc.org/github.com/h2non/bimg) for detailed API documentation.