A necessary tips for using latex added in README

[mryuans]

Motivation

When I first tried to use the LaTeX in my video, I constantly ran into a "LaTeX compilation failed" error. After read the tex_file_writing.py in manimlib/utils, I figured out that there is a necessary change for temporary file path, which is not obvious for users with their first trial. So I think it's better if it could be add to the README

Proposed changes

#### Tips for using LaTeX

Before using functions that need LaTeX, please change the  
temporary directory for intermediate files (which will lead to "LaTeX compilation failed")
you can change the `tex_file_writing.py` in `manimlib/utils':  
  ```tex_file_writing.py in full_tex_to_svg
  # Write intermediate files to a temporary directory
  with tempfile.TemporaryDirectory() as temp_dir: