DSK-658 - Add instruction how build mailcore for windows

[OleksiiShvachenko]

Note

Low Risk
Documentation-only change with no runtime or build script modifications.

Overview
Adds a new Updating mailcore2 on Windows (Spark prebuilt flow) section to README.md, placed after the platform build links and before Basic IMAP Usage.

The guide explains that Spark Windows builds consume S3 prebuilt C++ (mailcore2-all-<N>.zip via Get-Mailcore2.ps1), not source compiles, and walks through prerequisites (CI image, MSVC 14.39/STL1000 constraint, SDK 10.0.18362, Swift 5.10.1, ICU/libxml2 paths, SPARK_PREBUILT_KEY, private deps), building with build-windows-5.10\Build-Mailcore2.ps1 -Install, packaging the mailcore2-all zip layout, uploading to spark-prebuilt-binaries in eu-central-1, and rolling out via $PrebuiltMailcoreVersion, tags, and cross-repo pins in spark-core-mono. It also documents -BuildMailcore2 for local C++ testing without publishing a prebuilt.

^{Reviewed by Cursor Bugbot for commit 35726c2. Bugbot is set up for automated code reviews on this repo. Configure here.}