README.md: Add nesting modular/shell/exercises/genotype/

This gives the submodule-averse the nesting they'd usually get by
merging the master branch in a submodule at
modular/shell/exercises/genotype.

diff --git a/shell/exercises/1000gp.vcf b/modular/shell/exercises/genotype/1000gp.vcf
similarity index 100%
rename from shell/exercises/1000gp.vcf
rename to modular/shell/exercises/genotype/1000gp.vcf

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