add links to RTMP Errata and Addenda document

[zenomt]

This PR adds links to RTMP Errata and Addenda, a collection of corrections, clarifications, and amendments to the historical RTMP Specification of December 2012 (of which I am one of the listed authors/editors, though no longer at Adobe).

[zenomt]

note: at this time I'm still revising the Errata and Addenda document, and the intent is for it to be a "living document" (or at least more lively than the historical spec), so at this time a static copy into this website probably isn't warranted. However, it is MIT licensed and if the maintainers would prefer, you may instead bring in static copies of the renderings of the latest revision.

[ultrasaurus]

I think we should just go ahead and add the link. I would just suggest this be in a separate sub-section, since it's not officially from Adobe or Veriskope, nor is it peer reviewed. As currently positioned in the PR, it looks like an official spec document (especially since you are the author of the initially published spec, but this errata is not coming from your old official affiliation)

[zenomt]

I would just suggest this be in a separate sub-section

ok. do you think the "Related References" section is appropriate, or would you prefer a brand new section, such as "Unofficial References" or "Supplemental References"?

[zenomt]

nor is it peer reviewed

i have had a few rounds of review and revision out-of-band with @veovera, but additional review from others (especially yourself) would be greatly appreciated!

[zenomt]

i moved the link to a new "Other Resources" section at the end of the page.