Add appendix documenting legacy patterns for asynchronous operations

[markandrus]

In 2.1. One-and-Done Operations, the guide reads (emphasis mine)

Previously, web specifications used a large variety of differing patterns for asynchronous operations. We’ve documented these in an appendix below, so you can get an idea of what is now considered legacy. Now that we have promises as a platform primitive, such approaches are no longer necessary.

I don't believe such and appendix is included, and it would certainly be useful (e.g. analysis of benefits/tradeoffs).

[domenic]

Wow, good catch. These were moved in https://github.com/w3ctag/promises-guide/commit/8f3a099f28980e98b402577f86f7feb14bf4dc7c but then I think the Bikeshed conversion in https://github.com/w3ctag/promises-guide/commit/e3a29434fb6ec011b24e59ee52742cd03bbae068 lost them.

[markandrus]

Thanks!