Update error-codes.md

I'm not sure this is the right text, but it would have been great to have this option documented in association with the error to help people who encounter this (see dotnet#3145 (comment)).

docs/error-codes.md

@@ -35,6 +35,8 @@ the error code. For example:
 
 #### `IL1007`: Missing predefined 'type' type
 
+- Consider adding the missing reference to your project or pass `--disable-opt unreachablebodies` to ignore.
+
 #### `IL1008`: Could not find constructor on 'type'
 
 #### `IL1009`: Assembly reference 'name' could not be resolved
@@ -1979,4 +1981,4 @@ void TestMethod()
     // IL3003: Member 'Implementation.TestMethod()' with 'RequiresAssemblyFilesAttribute' implements interface member 'IRAF.TestMethod()' without 'RequiresAssemblyFilesAttribute'. For all interfaces and overrides the implementation attribute must match the definition attribute.
     public void TestMethod () { }
   }
-  ```
+  ```