Add a doc describing how to add new RIDs to source-build

Anyone else adding their Linux distribution's RID to source-build
might find this useful.

I am trying to get this out of my head and into a shape where others
can make use of this.

See: dotnet#2095

Documentation/add-new-rid.md

@@ -0,0 +1,137 @@
+# Adding a new Runtime Identifier (RID) to source-build
+
+This document describes how to add a new RID to source-build. You
+might find this useful if you see an error like this when building
+source-build:
+
+```
+error NETSDK1083: The specified RuntimeIdentifier 'foo.bar-x64' is not recognized.
+```
+
+# Assumptions
+
+- .NET (coreclr, source-build, etc) already supports this new OS.
+
+  For example, this could be a new version of a Linux distribution
+  that's already known to work. Or it could be a fork of another Linux
+  distribution which is expected to work
+
+- source-build computes the RID correctly
+
+  Source-build computes the RID, at least on Linux, by looking at
+  `/etc/os-release` and running `${VERSION}.${VERSION_ID}-$(uname
+  -m)`. If the result of this computation itself is wrong (for example
+  a rolling Linux distro doesn't use `VERSION_ID`), the steps in this
+  doc wont be sufficient.
+
+- We just need to add things to the RID graph
+
+# Step-by-step fix
+
+This error message is generally from the rolysn build:
+
+```
+error NETSDK1083: The specified RuntimeIdentifier 'foo.bar-x64' is not recognized.
+```
+
+It means that the SDK that's being used to build roslyn doesn't have
+the runtime id (RID) in it's RID graph. We can fix that.
+
+The RID is generally present in 2 places:
+
+1. `RuntimeIdentifierGraph.json` in a built-SDK
+
+2. `Microsoft.NETCore.Platforms/runtime.json` in the source
+   repositories (dotnet/runtime or dotnet/corefx)
+
+Generally fixing the second one is sufficient.
+
+Here's how to fix it.
+
+1. Grab the source code
+
+   For .NET Core (3.1 or earlier):
+
+   ```
+   git clone https://github.com/dotnet/corefx
+   cd corefx
+   git checkout release/$version
+   ```
+
+   For .NET (5 or later):
+
+
+   ```
+   git clone https://github.com/dotnet/corefx
+   cd corefx
+   git checkout release/$version
+   ```
+
+2. Add your new RID to `Microsoft.NETCore.Platforms`'s
+   `runtimeGroups.props` file
+
+   If you are adding a new Linux distro, the chagne might look
+   something like this:
+
+   ```diff
+   --- a/pkg/Microsoft.NETCore.Platforms/runtimeGroups.props
+   +++ b/pkg/Microsoft.NETCore.Platforms/runtimeGroups.props
+   @@ -40,6 +40,11 @@
+          <TreatVersionsAsCompatible>false</TreatVersionsAsCompatible>
+        </RuntimeGroup>
+
+   +    <RuntimeGroup Include="exherbo">
+   +      <Parent>linux</Parent>
+   +      <Architectures>x64</Architectures>
+   +    </RuntimeGroup>
+   +
+        <RuntimeGroup Include="fedora">
+          <Parent>linux</Parent>
+          <Architectures>x64;arm64</Architectures>
+   ```
+
+   If you are just adding a new version, it might look like this:
+
+   ```diff
+   --- a/pkg/Microsoft.NETCore.Platforms/runtimeGroups.props
+   +++ b/pkg/Microsoft.NETCore.Platforms/runtimeGroups.props
+   @@ -43,7 +43,7 @@
+        <RuntimeGroup Include="fedora">
+          <Parent>linux</Parent>
+          <Architectures>x64;arm64</Architectures>
+   -      <Versions>23;24;25;26;27;28;29;30;31;32;33;34</Versions>
+   +      <Versions>23;24;25;26;27;28;29;30;31;32;33;34;35</Versions>
+          <TreatVersionsAsCompatible>false</TreatVersionsAsCompatible>
+        </RuntimeGroup>
+   ```
+
+   Then build with the `/p:UpdateRuntimeFiles=true` flag:
+
+    ./build.sh /p:UpdateRuntimeFiles=true
+
+   Building should result in a patch that updates
+   `runtimeGroups.props`, but also `runtime.json` and
+   `runtime.compatiblity.json`.
+
+3. Commit the results and generate a patch:
+
+   ```
+   git commit -a ...
+   git format-patch -1 HEAD
+   ```
+
+   Here's an [example
+   result](https://src.fedoraproject.org/rpms/dotnet3.1/raw/43e957aabcbcef730c9433b817be947a7820ae67/f/corefx-43032-fedora-35-rid.patch)
+   of a real patch generated though this approach.
+
+4. Add that patch to source-build
+
+   For .NET Core 3.1 or earlier, add the patch to `patches/corefx/`
+
+   For .NET 5, add the patch to `patches/runtime/`
+
+5. Build source-build
+
+   Use your regular build steps, such as `./build.sh`. source-build
+   will apply the new patch and build a new SDK with the new RID
+   added.