Update README with GitHub Actions badge and concise description

Author: anthrotype

README.md

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-[![Build status](https://ci.appveyor.com/api/projects/status/tgxivwffrbvpxp5c/branch/master?svg=true)](https://ci.appveyor.com/project/fonttools/skia-builder/branch/master)
+# skia-builder
+
+[![Build + Deploy](https://github.com/fonttools/skia-builder/actions/workflows/ci.yml/badge.svg)](https://github.com/fonttools/skia-builder/actions/workflows/ci.yml)
+
+Builds Skia libraries for multiple platforms and architectures (static for macOS/Linux, shared for Windows).
+
+Used by [skia-pathops](https://github.com/fonttools/skia-pathops) to provide precompiled Skia binaries for Python wheels.
+
+## Releases
+
+CI builds are triggered on tags. Release assets include:
+- `libskia-mac-x64.zip`, `libskia-mac-arm64.zip`, `libskia-mac-universal2.zip`
+- `libskia-linux-x64.zip`, `libskia-linux-arm64.zip`
+- `libskia-win-x86.zip`, `libskia-win-x64.zip`