Add whitespace policy

Author: Matthias Koeppe

src/doc/en/developer/coding_basics.rst

@@ -27,6 +27,26 @@ all Python/Cython code that is included with Sage should adhere to the
 style conventions discussed in this chapter.
 
 
+.. _section-coding-general:
+
+General Code Style
+==================
+
+Use spaces instead of tabs for indentation.  The only exception is for
+makefiles, in which tabs have a syntactic meaning different from
+spaces.
+
+Do not add trailing whitespace.
+
+Sage provides editor configuration for Emacs, using the file
+``.dir-locals.el``, to use spaces instead of tabs.  Regarding trailing
+whitespace, see https://www.emacswiki.org/emacs/DeletingWhitespace
+for various solutions.
+
+If you use another editor, we recommend to configure it so you do not
+add tabs to files.
+
+
 .. _section-coding-python:
 
 Python Code Style