Add README for test (#566)

* Add README for test

* Apply review feedback

* Add description to test_dataloader

test/README.md

+# Torchaudio Test Suite
+
+## Structure of tests
+
+The following is an overview of the tests and related modules for `torchaudio`.
+
+### Purpose specific test suites
+
+#### Numerical compatibility agains existing software
+- [Librosa compatibility test](./test_librosa_compatibility.py)
+    Test suite for numerical compatibility against librosa.
+- [SoX compatibility test](./test_sox_compatibility.py)
+    Test suite for numerical compatibility against SoX.
+- [Kaldi compatibility test](./test_kaldi_compatibility.py)
+    Test suite for numerical compatibility against Kaldi.
+
+#### Result consistency with PyTorch framework
+- [TorchScript consistency test](./test_torchscript_consistency.py)
+    Test suite to check 1. if an API is TorchScript-able, and 2. the results from Python and Torchscript match.
+- [Batch consistency test](./test_batch_consistency.py)
+    Test suite to check if functionals/Transforms handle single sample input and batch input and return the same result.
+
+### Module specific test suites
+
+The following test modules are defined for corresponding `torchaudio` module/functions.
+
+- [`torchaudio.datasets`](./test_datasets.py)
+- [`torchaudio.functional`](./test_functional.py)
+- [`torchaudio.transforms`](./test_transforms.py)
+- [`torchaudio.compliance.kaldi`](./test_compliance_kaldi.py)
+- [`torchaudio.kaldi_io`](./test_kaldi_io.py)
+- [`torchaudio.sox_effects`](test/test_sox_effects.py)
+- [`torchaudio.save`, `torchaudio.load`, `torchaudio.info`](test/test_io.py)
+
+### Test modules that do not fall into the above categories
+- [test_dataloader.py](./test_dataloader.py)
+    Simple test for loading data and applying preprocessing.
+
+### Support files
+- [assets](./assets): Contain sample audio files.
+- [assets/kaldi](./assets/kaldi): Contains Kaldi format matrix files used in [./test_compliance_kaldi.py](./test_compliance_kaldi.py).
+- [compliance](./compliance): Scripts used to generate above Kaldi matrix files.
+
+
+## Adding test
+
+When you add a new feature(functional/transform), consider the following
+
+1. When you add a new feature, please make it Torchscript-able and batch-consistent unless it degrades the performance. Please add the tests to see if the new feature meet these requirements.
+1. If the feature should be numerical compatible against existing software (SoX, Librosa, Kaldi etc), add a corresponding test.
+1. If the new feature is unique to `torchaudio` (not a PyTorch implementation of an existing Software functionality), consider adding correctness tests (wheather the expected output is produced for the set of input) under the corresponding test module (`test_functional.py`, `test_transforms.py`).