Mention SpecAugment in doc

[mthrok]

📚 Documentation

TimeStretch, TimeMasking and FrequencyMasking are implementation of SpecAugment, but it is not immediately clear from the documentation.

We can add

1. Reference to the paper
2. Add example images

Note: when adding images to documentation, we need to make sure that

1. documentation is still comprehensive without the image
2. the colormap is friendly to the people with color vision deficiencies. (viridis should do)

[krishnakalyan3]

@mthrok I would like to work on this.

[mthrok]

Thanks @krishnakalyan3. Let me know if you need help.