mention fsproj files as an example of other dotnet reference files

add mention to the fact that cross language project reference can work within valid .NET project files, for example F#

Author: jkone27

docs/core/tools/dotnet-add-reference.md

@@ -29,6 +29,7 @@ The `dotnet add reference` command provides a convenient option to add project r
   <ProjectReference Include="app.csproj" />
   <ProjectReference Include="..\lib2\lib2.csproj" />
   <ProjectReference Include="..\lib1\lib1.csproj" />
+  <ProjectReference Include="..\lib3\lib3.fsproj" />
 </ItemGroup>
 ```
 
@@ -72,6 +73,12 @@ There's no CLI command to add a reference to an assembly that isn't in a project
   dotnet add app/app.csproj reference lib/lib.csproj
   ```
 
+- Add a compatible .NET language project (e.g. F#) referenc, works in both ways:
+
+  ```dotnetcli
+  dotnet add app/app.csproj reference lib/lib.fsproj
+  ```
+
 - Add multiple project references to the project in the current directory:
 
   ```dotnetcli