Add notes about the PuppetDB module to the README.
Add notes about the PuppetDB module to the README.

Our installation docs did not include any mention of using the
forge module to install; this commit adds some brief notes and
links.

shrug/puppetdb,grimradical/puppetdb,jantman/puppetdb,shrug/puppetdb,puppetlabs/puppetdb,rbrw/puppetdb,kbrezina/puppetdb,shrug/puppetdb,cprice404/puppetdb,melissa/puppetdb,nfagerlund/puppetdb,wkalt/puppetdb,ajroetker/puppetdb,highb/puppetdb,highb/puppetdb,johnduarte/puppetdb,johnduarte/puppetdb,highb/puppetdb,rbrw/puppetdb,mullr/puppetdb,senior/puppetdb,mullr/puppetdb,johnduarte/puppetdb,wkalt/puppetdb,mullr/puppetdb,nfagerlund/puppetdb,github/puppetlabs-puppetdb,mullr/puppetdb,melissa/puppetdb,kbarber/puppetdb,kbarber/puppetdb,nfagerlund/puppetdb,rbrw/puppetdb,ajroetker/puppetdb,waynr/puppetdb,grimradical/puppetdb,highb/puppetdb,github/puppetlabs-puppetdb,puppetlabs/puppetdb,grimradical/puppetdb,cprice404/puppetdb,mullr/puppetdb,senior/puppetdb,ajroetker/puppetdb,jantman/puppetdb,rbrw/puppetdb,github/puppetlabs-puppetdb,rbrw/puppetdb,puppetlabs/puppetdb,grimradical/puppetdb,puppetlabs/puppetdb,waynr/puppetdb,kbrezina/puppetdb,kbarber/puppetdb,ajroetker/puppetdb,waynr/puppetdb,waynr/puppetdb,jantman/puppetdb,kbarber/puppetdb,kbrezina/puppetdb,wkalt/puppetdb,senior/puppetdb,melissa/puppetdb,wkalt/puppetdb,senior/puppetdb,cprice404/puppetdb,johnduarte/puppetdb,kbrezina/puppetdb,shrug/puppetdb,puppetlabs/puppetdb