doc(README): document fetching Mathlib oleans

[cdbattags]

The "Using CSLib in your project" section (#206) stops at lake update cslib. The next lake build then compiles Mathlib. This adds lake exe cache get there, which is the same step Mathlib documents for downstream projects.

lake exe cache get does not keep CSLib's own build products. The README also notes LAKE_ARTIFACT_CACHE=true so another checkout of the same toolchain can reuse a local build, and LAKE_RESTORE_ARTIFACTS=true when a tool expects files under .lake/build. lake cache put is only described, not added to CI: it needs LAKE_CACHE_KEY and a cache service (LAKE_CACHE_SERVICE, or LAKE_CACHE_ARTIFACT_ENDPOINT with LAKE_CACHE_REVISION_ENDPOINT).

Drafted with Grok 4.7, the 500k-context high-effort setting, in Cursor. I used it to read LAKE_ARTIFACT_CACHE, LAKE_RESTORE_ARTIFACTS, and LAKE_CACHE_KEY in Lean 4.35 (Lake.Config.Env, PackageConfig.enableArtifactCache) and to write the README paragraph. This repository has no LLM-generated label; the disclosure above is the step CONTRIBUTING.md asks for, via the Mathlib policy.