Build instructions in readme (#56)

Readme.md

@@ -2,7 +2,7 @@
 
 A Swift library for reading and inspecting .NET metadata, including WinMD files, following the [ECMA-335, Common Language Infrastructure (CLI)](https://www.ecma-international.org/publications-and-standards/standards/ecma-335/) standard. Parsing IL is currently out of scope, but not off the table in the future.
 
-![example branch parameter](https://github.com/tristanlabelle/swift-dotnetmetadata/actions/workflows/build-and-test.yml/badge.svg?branch=main)
+![Build and Test workflow](https://github.com/tristanlabelle/swift-dotnetmetadata/actions/workflows/build-and-test.yml/badge.svg?branch=main)
 
 ## Example
 
@@ -38,6 +38,24 @@ let package = Package(
 )
 ```
 
+## Building
+Prerequisites:
+
+- A Windows OS (building on other platforms should be possible but is untested)
+- A Swift toolchain, version 5.8 or above
+- Visual Studio (any edition) with the C++ Workload
+- A Windows SDK (comes with Visual Studio)
+- CMake and ninja (comes with Visual Studio, or use `winget install Kitware.CMake Ninja-build.Ninja`)
+
+### In Visual Studio Code
+Open Visual Studio Code from a `x64 Native Tools Command Prompt for VS 2022` and use standard IDE commands for building (<kbd>ctrl</kbd>+<kbd>shift</kbd>+<kbd>B</kbd>).
+
+### From the command line
+This project builds with both the Swift Package Manager (SPM) and CMake. SPM supports running tests, but requires a helper script because it cannot integrate all build steps.
+
+1. Open a `x64 Native Tools Command Prompt for VS 2022`
+2. Refer to the [Build and Test workflow](.github/workflows/build-and-test.yml) for build commands
+
 ## Architecture
 
 The library consists in two layered modules: