README.md: Add nesting modular/

This gives the submodule-averse the nesting they'd usually get by
merging the master branch in a submodule at modular/.  They'll have to
merge the submodule's 'namespaced' branches by hand to pull in the
equivalent content.

diff --git a/README.md b/modular/README.md
similarity index 100%
rename from README.md
rename to modular/README.md