add initial Makefile

The initial Makefile only supports `make init`, which will make sure all
submodules are cloned and have reasonable remotes configured.

`make init` is not yet idempotent so is not safe to run multiple times.
add brief Setup section to README
bugfix:  verify parameters for top level remote
allow arbitrarily named push/pull remotes

Previously, the pull remote could not be safely named `origin`.
`make init` is idempotent
submodule targets need to verify github params
set git config defaults for pushing
extract repo setup into a function
remove unneeded relative path
remove _NAME suffix for push/pull remote variables
git-fetch after repo setup
add `all` target which prints basic help
add `help` target
don't require a github account or forks in `make init`
use `notdir` instead of `lastword` + `subst`
Merge pull request #1 from mark-burnett/add-makefile

Add initial Makefile