Mention the DotnetFscCompilerPath property (#13874)

The very useful `DotnetFscCompilerPath` property doesn't seem to be documented anywhere. I'm not sure if there's an authoritative location for .fsproj properties, but it might be useful to mention this property in the DEVGUIDE.

Author: tboby

DEVGUIDE.md

@@ -121,6 +121,18 @@ You can find all test options as separate flags. For example `build -testAll`:
 
 Running any of the above will build the latest changes and run tests against them.
 
+## Using your custom compiler to build other projects
+
+Building the compiler using `build.cmd` or `build.sh` will output artifacts in `artifacts\bin`. 
+
+To use your custom build of `Fsc`, add the `DotnetFscCompilerPath` property to your project's `.fsproj` file, adjusted to point at your local build directory, build configuration, and target framework as appropriate:
+
+```xml
+<PropertyGroup>
+    <DotnetFscCompilerPath>D:\Git\fsharp\artifacts\bin\fsc\Debug\net7.0\fsc.dll</DotnetFscCompilerPath>
+</PropertyGroup>
+```
+
 ## Updating FSComp.fs, FSComp.resx and XLF
 
 If your changes involve modifying the list of language keywords in any way, (e.g. when implementing a new keyword), the XLF localization files need to be synced with the corresponding resx files. This can be done automatically by running