Update cycle

Release cycles in Lean are a mess right now, especially for packages downstream of Mathlib. Expect this document to change as we figure out what users need.

Branches:

• main is staying on the most recent stable release of the compiler. This is for users who don't want to be on compiler release candidates.
• mathlib-nightly tracks Mathlib master branch each night, which right now tracks compiler release candidates. This is expected to break fairly often; I will try to keep it compiling.

Releases:

• For each stable release, we tag the earliest main commit with that release in its lean-toolchain file (which is, usually, just the commit that bumps the lean-toolchain file). These tags never move.