User documentation for `colony_mask_features` scripts

Currently the README.md file does not mention any of the scripts in the `colony_mask_features` folder. If we want users to discover and use those, we need to write instructions in the top-level README.

There is a README at `EMT_data_analysis/colony_mask_features/README.md` that the top-level README can reference. In addition, I expect users would benefit from context on how the `colony_mask_features` scripts fit in the context of this repo & the paper.