Add nms compile issue to trouble_shooting page (#733)

* Add nms compile issue to trouble_shooting page

* after lint

* update link to online doc

README.md

@@ -171,3 +171,8 @@ pip install mmcv-full
 Note that the local compiling may take up to 10 mins.
 
 If you would like to build MMCV from source, please refer to the [guide](https://mmcv.readthedocs.io/en/latest/build.html).
+
+## FAQ
+
+If you face some installation issues, CUDA related issues or RuntimeErrors,
+you may first refer to this [Trouble Shooting Page](https://mmcv.readthedocs.io/en/latest/trouble_shooting.html).

docs/trouble_shooting.md

@@ -35,3 +35,8 @@ Feel free to enrich the list if you find any frequent issues and have ways to he
 
     This error may be due to your poor GPU. Try to decrease the value of [THREADS_PER_BLOCK](https://github.com/open-mmlab/mmcv/blob/cac22f8cf5a904477e3b5461b1cc36856c2793da/mmcv/ops/csrc/common_cuda_helper.hpp#L10)
     and recompile mmcv.
+
+- "RuntimeError: nms is not compiled with GPU support".
+
+    This error is because your CUDA environment is not installed correctly.
+    You may try to re-install your CUDA environment and then delete the build/ folder before re-compile mmcv.