add finetune from detectron guide (#286)

README.md

@@ -192,6 +192,14 @@ While the aforementioned example should work for training, we leverage the
 cocoApi for computing the accuracies during testing. Thus, test datasets
 should currently follow the cocoApi for now.
 
+## Finetuning from Detectron weights on custom datasets
+Create a script `tools/trim_detectron_model.py` like [here](https://gist.github.com/wangg12/aea194aa6ab6a4de088f14ee193fd968).
+You can decide which keys to be removed and which keys to be kept by modifying the script.
+
+Then you can simply point the converted model path in the config file by changing `MODEL.WEIGHT`.
+
+For further information, please refer to [#15](https://github.com/facebookresearch/maskrcnn-benchmark/issues/15).
+
 ## Troubleshooting
 If you have issues running or compiling this code, we have compiled a list of common issues in
 [TROUBLESHOOTING.md](TROUBLESHOOTING.md). If your issue is not present there, please feel