Document new optional configuration file name

Document the feature introduced by:
dlang/dmd#2095

dcompiler.dd

@@ -768,6 +768,11 @@ $(WINDOWS
         $(LI directory $(B dmd.exe) resides in)
         )
 
+	Additionally, starting from DMD 1.077 and to ease D1 to D2 migration,
+	it will first look for a file $(D sc1.conf) in the same locations
+	described above. This way you can have both dmd1 and dmd2 installed and
+	use $(D sc1.conf) for D1 and $(D sc.conf) for D2.
+
         $(P If found, environment variable
         settings in the file will override any existing settings.
         This is handy to make $(B dmd) independent of programs with
@@ -838,6 +843,11 @@ $(UNIX
         $(LI $(D /etc/))
         )
 
+	Additionally, starting from DMD 1.077 and to ease D1 to D2 migration,
+	it will first look for a file $(D dmd1.conf) in the same locations
+	described above. This way you can have both dmd1 and dmd2 installed and
+	use $(D dmd1.conf) for D1 and $(D dmd.conf) for D2.
+
         $(P If found, environment variable
         settings in the file will override any existing settings.
         This is handy to make $(B dmd) independent of programs with