Update HACKING file with coding style hints and tips

HACKING

@@ -20,6 +20,22 @@ Curtis
 
 
 
+Coding style
+============
+
+The GParted code is all over the place in terms of coding style.  New
+code, and updated code where reasonable, should use Allman style and
+Smart Tabs alignment.  See src/OptionComboBox.cc and
+src/SupportedFileSystems.cc as examples.  Don't worry too much about the
+style, we can always adjust it if necessary during the review and merge
+process.
+
+* https://en.wikipedia.org/wiki/Indentation_style#Allman_style
+* https://www.emacswiki.org/emacs/SmartTabs
+* https://vim.fandom.com/wiki/Indent_with_tabs,_align_with_spaces
+
+
+
 Translatable files
 ==================