Add Makefile documentation

[ddelange]

Motivation

kserve/modelmesh-runtime-adapter#38 (comment)

Modifications

Add help texts

Result

all                       - Alias for `build`
build                     - Build runtime docker image
develop                   - Build develop docker image and run an interactive shell in the develop envionment
fmt                       - Run formatting
help                      - Print Makefile documentation
run                       - Build develop docker image and run a make command in the develop envionment (e.g. `make run fmt` will execute `make fmt` within the docker container)
test                      - Run tests

[ddelange]

cc @ckadner this is the last one I think

[ckadner]

cc @ckadner this is the last one I think

Thanks @ddelange I will add my review comments shortly.

[ckadner]

This is great! Thanks @ddelange

/lgtm