(Maint) Remove core function documentation from README
(Maint) Remove core function documentation from README

Without this patch the README contains the documentation for core
functions shipped in Puppet in addition to the functions shipped in
stdlib.

This is a problem because it's confusing for end users trying to get
started with puppet.

This patch makes it so only the stdlib functions are included.

yadavnikhil/puppetlabs-stdlib,jbondpdx/puppetlabs-stdlib,fuel-infra/puppetlabs-stdlib,HelenCampbell/puppetlabs-stdlib,puppetlabs/puppetlabs-stdlib,DavidS/puppetlabs-stdlib