Style Guidelines improvement, thank you Alfredo

DEVELOPER_GUIDELINES.md

@@ -60,7 +60,11 @@ The following is a non-exhaustive list of the development guidelines.
 
 ### Style
 
-When we code, we are using the [common Haskell Style guide](https://github.com/tibbe/haskell-style-guide/blob/master/haskell-style.md).
+When we code, we try to use the [common Haskell Style guide](https://github.com/tibbe/haskell-style-guide/blob/master/haskell-style.md).
+
+1. For new files, use the referenced style guide;
+2. For older files, which might have been written using a different code style, try to respect whichever style guide was used to write the file (to ensure consistency and minimise unwanted changes);
+3. Resist the urge of making style modifications mixed to general refactoring; rather separate those into independent commits, so that they are easy to revert if unwanted / not needed
 
 ### Code Of Conduct