Simplify README.md generation

Currently, README.mds are generated via `readme_gen` which actually does more than generating README.mds, for example, it also does linting.

We probably want to factor the README.md generation into a separate file from which we can generate one binary that is easy to use. For example, it should be easy to generate the README.md for the entire project and for directories—perhaps this can be achieved by having a simple binary + basic Unix utilities.

It would also be nice if it were written in a way that would allow for the boilerplate in `Holmakefiles` to be removed. I am talking about things such as:
```
all: $(DEFAULT_TARGETS) README.md
.PHONY: all

README_SOURCES = $(wildcard *Script.sml) $(wildcard *Lib.sml) $(wildcard *Syntax.sml)
DIRS = $(wildcard */)
README.md: $(CAKEMLDIR)/developers/readme_gen readmePrefix $(patsubst %,%readmePrefix,$(DIRS)) $(README_SOURCES)
$(protect $(CAKEMLDIR)/developers/readme_gen) $(README_SOURCES)
```