Write docs on porting code to Trio

There should be a section in the docs explicitly addressing the question: I need a library for Trio; it doesn't exist yet; what are my options?

Maybe the best arrangement is as a set of brief case studies, looking at how different libraries have handled this, and the tradeoffs involved in each choice. (What does your library do, do you care about sync, do you care about asyncio, do you care about taking advantage of native trio stuff, etc.)

Strategies to discuss (possibly incomplete):

• native Trio code (example: trio-websocket)
  • possibly with sans-I/O
  • special case for HTTP API wrappers (examples: gidgethub, this article from @pgjones's colleague)
• trio-asyncio:
  • to use existing asyncio libraries
  • to incrementally port an application from asyncio to trio
• unasync for "essentially sync" libraries:
  • with multiple I/O backends (urllib3)
  • with no backend b/c you're a middle-layer that doesn't do I/O anyway (flask)
• anyio

Start by reviewing the existing Trio documentation and the issue's listed examples and strategies, including trio-websocket, trio-asyncio, unasync, and anyio. Done should be a clearly organized section explaining the available porting options, with brief case studies and the relevant tradeoffs for sync support, asyncio compatibility, and native Trio integration.