Implement readme_gen in CakeML

This issue is similar to #380 and #381, except for the `developers/readme_gen.sml` program that does linting and generates `README.md` files. Ideally, the implementation would be generated in part from HOL functions that are translated into CakeML (and about which we can prove some correctness properties). However, some bootstrapping may be required (since the `readme_gen` program should be run before much of the CakeML ecosystem is built). I might also propose to rename the program to `lint` or similar.