Update document for unittest and CONTRIBUTING.md

Author: yangarbiter

CONTRIBUTING.md

@@ -55,9 +55,16 @@ conda install pytorch -c pytorch-nightly
 pip install cmake ninja
 ```
 
+### Install other requirements
+
+```bash
+pip install -r requirements.txt
+```
+
 ```bash
 git clone https://github.com/pytorch/audio.git
 cd audio
+git submodule sync
 git submodule update --init --recursive
 BUILD_SOX=1 python setup.py develop
 # or, for OSX

test/torchaudio_unittest/README.md

@@ -9,6 +9,7 @@ For testing, the following packages are needed:
 
 ```bash
 pip install typing pytest scipy numpy parameterized
+pip install -r requirements.txt
 ```
 
 Make sure to follow the installation instruction in the [contributing
@@ -73,7 +74,9 @@ The following test modules are defined for corresponding `torchaudio` module/fun
 - [`torchaudio.compliance.kaldi`](./compliance_kaldi_test.py)
 - [`torchaudio.kaldi_io`](./kaldi_io_test.py)
 - [`torchaudio.sox_effects`](./sox_effect)
-- [`torchaudio.save`, `torchaudio.load`, `torchaudio.info`](./io_test.py)
+- [`torchaudio.save`, `torchaudio.load`, `torchaudio.info`]
+    - soundfile backend [[save](./backend/soundfile/save_test.py), [load](./backend/soundfile/load_test.py), [info](./backend/soundfile/info_test.py)]
+    - SoX backend [[save](./backend/sox_io/save_test.py), [load](./backend/sox_io/load_test.py), [info](./backend/sox_io/info_test.py)]
 
 ### Test modules that do not fall into the above categories
 - [test_dataloader.py](./dataloader_test.py)