Add CONTRIB.md

[vincentqb]

We should add a CONTRIB.md file to empower users in contributing more.

cc comment, T57210445

[vincentqb]

Thoughts on the following? CONTRIB.md should

• open an issue first to get feedback before coding :)
• test for correctness against librosa/sox/scipy/kaldi/etc
• test for torchscriptability on CPU and GPU, if possible
• test the operation is batchable
• add documentation
• avoid additional dependencies (e.g. numpy, scipy, etc)

see also README for tests for more details.

[bhargavkathivarapu]

@vincentqb , I would like to help with this . How do you want to organize this ?
I am thinking of below organization. thoughts ?

1. Basic setups & installations
2. Some resources to understand pytorch tensors
3. A generic workflow from raise issue , code change , code linting , submitting PR , merging
4. Then some different ways of contributing and details,checklist specific to each type of contribution
   • Contribute to Datasets
   • Contribute to Transforms
   • Contribute to Functional
   • Contribute to Examples
   • Contribute to documentation
   • Other contributions