Add instructions for making idl file changes (#18188)

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+# Updating idl files
+
+This directory has a variety of .idl files (such as corprof.idl) that need a little special handling when you make changes. Originally when we built on Windows only
+the build rules would automatically convert the idls into corresponding .h/.c files and include them in compilations. On non-windows platforms we don't have an equivalent
+for midl.exe which did that conversion so we work around the issue by doing:
+
+- Build on Windows as normal, which will generate files in bin\obj\Windows_NT.x64.Debug\src\inc\idls_out\
+- Copy any updated headers into src\pal\prebuilt\inc\
+- If needed, adjust any of the .cpp files in src\pal\prebuilt\idl\ by hand, using the corresponding bin\obj\Windows_NT.x64.Debug\src\inc\idls_out\*_i.c as a guide. Typically
+this is just adding MIDL_DEFINE_GUID(...) for any new classes/interfaces that have been added to the idl file.
+
+Include these src changes with the remainder of your work when you submit a PR.
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