README.md: Remove nesting shell/exercises/

There's no need to hard-code target directories in this repository.
Anyone consuming this material can use submodules and pick their own
target directory.

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

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