Updated README.md to document inclusion of default recipe for LWRP

Continuing thread from issue: https://github.com/onehealth-cookbooks/sysctl/issues/9

Admittedly it does make sense to always persist sysctl settings, and I can't think of a use case why you wouldn't want to. I brought up this issue more as general idea to clear up some confusion around the documentation.

Generally, when using LWRPs, you don't need to include recipes for them to work. However, in the `sysctl` cookbook's case, there's a hard dependency on the recipe, so the examples should reflect this.