Add fetch-submodules to main makefile make help

[TG-Techie]

This adds the new suggested method for downloading submodules into the make help command.

Rationale

After returning to (the internals of) CircuitPython, I forgot the newer command for initing the submodules. It looks like the documentation is only in the circuitpython build guide.

Output

tg-techie@ 12May2022 circuitpython % make help
Please use `make <target>' where <target> is one of
  fetch-submodules      to fetch dependencies from submodules, run this right after you clone the repo
  html       to make standalone HTML files

remaining

• Should the description convey anything else? less?

Side Note

It's great to see all the tooling improvements😊. I look forward to more; hug report hugs all around, and thank you.

[jepler]

Thanks!