Add README with instructions

README.md

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+TorchCodec
+==========
+
+Welcome to TorchCodec!
+
+TODO: Write a decent readme. For now I'm just writing basic info for OSS 
+development.
+
+
+Installing from source
+----------------------
+
+Use a conda or virtual environment.
+
+Install torch nightly, e.g.:
+
+```bash
+pip install --pre torch --index-url https://download.pytorch.org/whl/nightly/cpu
+```
+
+For more options, e.g. if you need CUDA or prefer using `conda`, refer to the
+[official page](https://pytorch.org/get-started/locally/).
+
+```bash
+BUILD_AGAINST_ALL_FFMPEG_FROM_S3=1 pip install -e ".[dev]" --no-build-isolation -vvv
+```
+
+If you prefer building against an installed version of FFmpeg you can ommit the
+`BUILD_AGAINST_ALL_FFMPEG_FROM_S3=1` part. Make sure `pkg-config` is installed
+and able to find your FFmpeg installation. The easiest way to do this is to
+install both from conda:
+
+```bash
+conda install ffmpeg pkg-config -c conda-forge
+BUILD_AGAINST_ALL_FFMPEG_FROM_S3=1
+```
+
+Building the docs
+-----------------
+
+First install from source, then install the doc dependencies:
+
+```bash
+cd docs
+pip install -r requirements.txt
+```
+
+Then, still from within the `docs` directory:
+
+```bash
+make html
+```
+
+Run `make clean` from time to time if you encounter issues.