Don't trim trailing whitespace on .rst files

Our existing .rst files have lines with trailing whitespace in them,
which I can only assume is deliberate, so update the editorconfig to
stop trimming trailing whitespace for these files.

.editorconfig

@@ -12,7 +12,7 @@ max_line_length = 100
 [{Makefile,*.in}]
 indent_style = tab
 
-[*.md]
+[*.{md,rst}]
 trim_trailing_whitespace = false
 
 [*.{sh,ac}]