Add documentation (or link) for LTS release channel

📖 Missing or Out-of-Date Documentation
We recently announced via the [AMP Blog](https://blog.amp.dev/2020/02/05/behind-the-scenes-deploying-the-amp-runtime/) a new LTS release channel, which has a slower (~monthly) release cadence. This is primarily for publishers that need to perform QA cycles on each release who prefer more infrequent updates/a more stable runtime. We have some documentation in ampproject/amphtml on the [LTS release](https://github.com/ampproject/amphtml/blob/master/contributing/lts-release.md) and the [release schedule](https://github.com/ampproject/amphtml/blob/master/contributing/release-schedule.md), but those and the blog post aren't likely to get much visibility to new publishers who aren't explicitly looking for it. The documentation (such as [here](https://amp.dev/documentation/guides-and-tutorials/start/create/basic_markup/?format=websites) or [here](https://amp.dev/documentation/guides-and-tutorials/learn/spec/amphtml/?format=websites)) should either describe the LTS release or link to one of the above linked documentation sources.

/cc @nainar @sebastianbenz