Improve preview release documentation

Author: TeofilC

CONTRIBUTING.md

@@ -337,6 +337,25 @@ Currently, [@emilypi](https://github.com/emilypi), [@fgaz](https://github.com/fg
 `haskell.org/cabal`, and [@Mikolaj](https://github.com/Mikolaj) is the point of contact for getting
 permissions.
 
+Preview Releases
+----------------
+
+We make preview releases available to facilitate testing of development builds.
+
+Artifacts can be found on the [`cabal-head` release page](https://github.com/haskell/cabal/releases/tag/cabal-head).
+Each CI pipeline generates tarballs with a `cabal` executable, and pipelines on `HEAD` also update this release.
+
+We currently make available builds for:
+  - Linux, dynamically linked (requiring `zlib`, `gmp`, `glibc`)
+  - Linux, statically linked
+  - MacOS
+  - Windows
+
+The statically linked Linux executables are built using Alpine.
+To reproduce these locally, set up an Alpine build environment using GHCup,
+and then build by calling `cabal build cabal-install --enable-executable-static`.
+
+
 API Documentation
 -----------------
 

README.md

@@ -33,17 +33,24 @@ Ways to get the `cabal-install` binary
 _Getting unreleased versions of `cabal-install`_: gives you a chance to try out yet-unreleased features.
 Currently, we only provide binaries for `x86_64` platforms.
 
+#### Preview Releases
+
 1. _GitHub preview release built from the tip of the `master` branch_: [download from GitHub](https://github.com/haskell/cabal/releases/tag/cabal-head) or use this GHCup command to install:
 
     ```
     ghcup install cabal -u https://github.com/haskell/cabal/releases/download/cabal-head/cabal-head-Linux-x86_64.tar.gz head
     ```
 
     Replace "Linux" with "Windows" or "macOS" as appropriate.
+    
+    The default Linux build is dynamically linked against `zlib`, `gmp` and `glibc`.
+    You will need to have appropriate versions of these libraries installed to use it.
+    Alternatively a statically linked "Linux-static" binary is also provided.
 
 
 2. Even more cutting-edge binaries built from pull requests are always available
-   from the `Validate` worklow page on GitHub, at the very bottom of the page.
+   from the `Validate` worklow page on GitHub, at the very bottom of the page,
+   or from the `build-alpine` workflow for statically linked Linux builds.
 
 Ways to build `cabal-install` for everyday use
 --------------------------------------------