Remove trailing spaces

The commands

find . -type f -name '*.md' -exec sed --in-place 's/[[:space:]]\+$//' {} \+

and

find . -type f -name '*.tex' -exec sed --in-place 's/[[:space:]]\+$//' {} \+

were used to do so.

CONTRIBUTING.md

@@ -10,6 +10,7 @@
        this should include the rendered image. For documents, there should be
        some example text explaining what it is about.
 3. Use spaces for indenting.
+4. Don't have trailing spaces.
 
 
 ## Commit messages