(PDOC-13) Add documentation examples to README

In order to make it easier for users, give some basic examples
in the README of how write documentation that is compatible with
strings. Also provide a few resources for getting started.
(maint) Exclude files that cause YARD errors

Running `puppet strings server` generates documentation for all
modules in your modulepath, meaning that it also generates HTML
for strings itself. Since there are parts of strings that are not
meant to be documented with YARD, exclude the files that cause
error messages so that `puppet strings server` is not needlessly
noisy.