Add PAL_DBG_CHANNELS note

docs/debugging/README.md

@@ -48,3 +48,13 @@ corehost
 
 The native executable prouduced by .NET CLI will produce trace output
 if launched with `COREHOST_TRACE=1 ./powershell`.
+
+CoreCLR PAL
+-----------
+
+The native code in the CLR has debug channels to selectively output
+information to the console. These are controlled by the
+`PAL_DBG_CHANNELS`, e.g., `export PAL_DBG_CHANNELS="+all.all"`, as
+detailed in the `dbgmsg.h` [header][].
+
+[header]: https://github.com/dotnet/coreclr/blob/release/1.0.0-rc2/src/pal/src/include/pal/dbgmsg.h