[torchcodec] Add contributing guide (#66)

Summary:
Pull Request resolved: #66

I copied the relevant sections from torchvision:

https://github.com/pytorch/vision/blob/main/CONTRIBUTING.md

Reviewed By: NicolasHug

Differential Revision: D59344144

fbshipit-source-id: ad0bf8dc8a496ad9b88f4fc57f050b00b53f31fe

CONTRIBUTING.md

@@ -0,0 +1,79 @@
+# Contributing to TorchCodec
+
+You can contribute to this project by writing code, fixing issues or simply by
+using the library and reporting your feedback.
+
+## Development installation
+
+### Dependencies
+
+Start by installing the **nightly** build of PyTorch following the
+[official instructions](https://pytorch.org/get-started/locally/). Note that the
+official instructions may ask you to install TorchCodec itself. If you are doing
+development on TorchCodec, you should not install prebuilt TorchCodec packages.
+
+To build, run and test locally you will need the following packages
+(dependencies):
+
+1. C++ compiler+linker (g++) and C++ runtime library (libstdc++). This is
+   typically available on a baseline Linux installation already.
+1. python
+1. cmake
+1. pkg-config
+1. libtorch (this is part of Pytorch)
+1. ffmpeg
+1. pytest (for testing)
+
+You can install these using your favorite package manager. Example, for conda
+use:
+
+```bash
+# After installing the nightly pytorch version run this:
+conda install cmake pkg-config ffmpeg pytest -c conda-forge
+```
+
+### Clone and build torchcodec
+
+```bash
+git clone https://github.com/pytorch-labs/torchcodec.git
+cd torchcodec
+
+pip install -e ".[dev]" --no-build-isolation -vv
+
+# This should decode the sample video and generate a png file that is a frame in the video.
+python test/decoders/manual_smoke_test.py
+```
+
+### Running unit tests
+
+To run python tests run:
+
+```bash
+pytest test -vvv
+```
+
+To run C++ tests run:
+
+```bash
+mkdir build
+cd build
+cmake -DCMAKE_BUILD_TYPE=Debug -DBUILD_TESTS=1 -DCMAKE_PREFIX_PATH=$(python3 -c 'import torch;print(torch.utils.cmake_prefix_path)) ..
+cmake --build . -- VERBOSE=1
+ctest --rerun-failed --output-on-failure
+```
+
+## Development Process
+
+TBD
+
+### Code formatting and typing
+
+TBD
+
+## License
+
+By contributing to TorchCodec, you agree that your contributions will be
+licensed under the LICENSE file in the root directory of this source tree.
+
+Contributors are also required to
+[sign our Contributor License Agreement](https://code.facebook.com/cla).