Be more explicit that this site does not have feature parity with RITlug's

[ct-martin]

From comment in FOSSRIT/runbook#30

As I'm reading this I'm also realizing the code functionality is no longer the same as the RITlug site from combination of things that weren't merged to the FOSS site after being added to the RITlug site as well as changes for FOSS-specific things that didn't go upstream to the RITlug site. It would be worth explicitly mentioning that somewhere to prevent confusion.

Some specific examples for reference:

• RITlug has a post-event layout that uses extra metadata for a nicer displaying of events
  • There are some accompanying template changes for this too (beyond the layout)
• FOSSRIT removed the talk layout & tweaked other templates as a result
• RITlug has a calendar page (PR Add Calendar page #85 for merging here)
• RITlug uses metadata more heavily as a result
  • This matters for docs

To be clear, I'm not looking to completely dissociate the sites. This isn't too big of an issue, but as the sites are heavily related in code, it would be helpful to have noted in CONTRIBUTING.md for anyone who is contributing to both sites (particularly since the README.md notes this site is based on the RITlug site).

[ct-martin]

(Also, if you have a better title for the issue, please feel free to change)