Add how to documentation for stage 2 builds

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Documentation/how-to-stage2-build.md

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+# How to run a stage 2 build
+
+This document describes how to perform a stage 2 source build.
+
+## What is a stage 2 build
+
+A **stage 2 build** is when you run a source build, then take the SDK and packages produced by that build and use them to rebuild the product.
+Stage 2 builds are essential for validating a source‑built product.
+
+It’s common for stage 2 builds to surface issues.
+For example, if a new Roslyn analyzer is included in the SDK, it can introduce new build errors during the stage 2 build.
+These issues must be resolved for the product to be considered **source‑buildable**.
+
+The process of using built SDK and artifacts to rebuild the product is often called **bootstrapping**.
+This helps to ensure that the second build was produced using only sources on disk without any external inputs.
+Distribution maintainers use this process to initiate the build process: build once using the Microsoft SDK and artifacts, then rebuild using the resulting SDK and artifacts.
+
+## Steps to run a stage 2 build
+
+1. Run a stage 1 build
+
+    ```bash
+    git clone https://github.com/dotnet/dotnet.git dotnet1
+    pushd dotnet1
+    ./prep-source-build.sh
+    ./build.sh -sb
+    popd
+    ```
+
+2. Run a stage 2 build
+
+    ``` bash
+    mkdir -p stage1/sdk stage1/packages
+
+    tar -xf dotnet1/artifacts/assets/Release/Private.SourceBuilt.Artifacts.*.tar.gz -C stage1/packages
+
+    tar -xf dotnet1/artifacts/assets/Release/Sdk/*/dotnet-sdk-*.tar.gz -C stage1/sdk
+
+    git clone https://github.com/dotnet/dotnet.git dotnet2
+    pushd dotnet2
+    ./build.sh -sb --with-packages stage1/packages --with-sdk stage1/sdk
+    popd
+    ```
+
+    > **Note:** `prep-source-build.sh` is optional for stage 2. It is only needed if you want to remove checked‑in binaries before building.