doc: guidelines: add a note about ascii and emojis

Add a note about source files encoding and emojis.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>

doc/contribute/guidelines.rst

@@ -459,6 +459,8 @@ exceptions:
   comment, ``//``, is not allowed.
 * Use ``/**  */`` for doxygen comments that need to appear in the documentation.
 * Avoid using binary literals (constants starting with ``0b``).
+* Avoid using non-ASCII symbols in code, unless it significantly improves
+  clarity, avoid emojis in any case.
 
 When there are differences between the guidelines above and the formatting
 generated by code formatting tools, the guidelines above take precedence.