Add sphinxcontrib spelling instructions (#488)

Summary:
## Types of changes

- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing functionality to change)
- [x] Docs change / refactoring / dependency upgrade

## Motivation and Context / Related issue

Added instructions on how to perform spell checks on documentations built through Sphinx. The package is called [sphinxcontrib-spelling](https://sphinxcontrib-spelling.readthedocs.io/en/latest/index.html).

Related to #380

## How Has This Been Tested (if it applies)

Documentation update

## Checklist

- [x] The documentation is up-to-date with the changes I made.
- [x] I have read the **CONTRIBUTING** document and completed the CLA (see **CONTRIBUTING**).
- [ ] All tests passed, and additional code has been covered with new tests.

Pull Request resolved: #488

Reviewed By: ffuuugor

Differential Revision: D39118347

Pulled By: zycalice

fbshipit-source-id: 148c936e4faad2b181509380d68607923dc91738

CONTRIBUTING.md

@@ -95,6 +95,32 @@ Run following command from `website` folder. It will build the docs and serve th
 ./scripts/build_website.sh
 ```
 
+You can also perform spell checks on documentation automatically (besides IDEs) using [```sphinxcontrib-spelling```](https://sphinxcontrib-spelling.readthedocs.io/en/latest/install.html)
+Note that you will also need [```PyEnchant```](https://pyenchant.github.io/pyenchant/) to run ```sphinxcontrib-spelling```, and thus the Enchant C library. Use this guide for ```PyEnchant```. 
+
+Steps:
+1. Install the extension with pip: ```pip install sphinxcontrib-spelling```
+2. Add ```sphinxcontrib.spelling``` to the extensions list in ```conf.py```.
+3. Install ```PyEnchant```. Please follow the [installation guide](https://pyenchant.github.io/pyenchant/install.html). Noticed that Apple Silicons may require a way around under section "Apple Silicon related errors".
+4. Make sure you have a ```source``` and ```build``` folder. Pass "spelling" as the builder argument to ```sphinx-build```.
+   ```
+   cd website/sphnix
+   mkdir build  # if you do not already have one
+   sphinx-build -b spelling source build
+   ```
+5. Find files with spelling errors in ```build``` (remember to check each folder). A file will be generated for each source file that contains spelling error. Example:
+   * File name: ```batch_memory_manager.spelling```
+   * File content:
+   ```
+   ../../opacus/utils/batch_memory_manager.py:docstring of opacus.utils.batch_memory_manager.BatchMemoryManager:5: (occasinal)  safeguarding against occasinal large batches produced by
+   ../../opacus/utils/batch_memory_manager.py:docstring of opacus.utils.batch_memory_manager.BatchMemoryManager:13: (optimzer)  On every step optimzer will check if the batch was the last physical batch comprising
+   ../../opacus/utils/batch_memory_manager.py:docstring of opacus.utils.batch_memory_manager.BatchMemoryManager:14: (behaviour)  a logical one, and will change behaviour accordignly.
+   ../../opacus/utils/batch_memory_manager.py:docstring of opacus.utils.batch_memory_manager.BatchMemoryManager:14: (accordignly)  a logical one, and will change behaviour accordignly.
+   ../../opacus/utils/batch_memory_manager.py:docstring of opacus.utils.batch_memory_manager.BatchSplittingSampler:4: (physocal)  Used to split large logical batches into physocal batches of a smaller size,
+   ```
+6. Manually review the spelling files and make changes in source files accordingly. Some detections are not perfect. For example, "nn" (from torch.nn) can be detected as a spelling error.
+
+
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