[docs] Missing documentation dependency 'autodocsumm' on docs/README.txt

[leandron]

When testing a new tutorial I'm writing about TVMC, I noticed that the guidance to generate docs is missing one plugin: autodocsumm.

If this is not present you'll see an error like this:

$ ./tests/scripts/task_sphinx_precheck.sh
cd python; python3 setup.py build_ext --inplace
WARNING: Cython is not installed, will compile without cython module
running build_ext
PreCheck sphinx doc generation WARNINGS..
rm -rf _build/*
rm -rf gen_modules
rm -rf tutorials
rm -rf vta/tutorials
python3 -m sphinx -b html -d _build/doctrees   . _build/html
Running Sphinx v3.2.1

Extension error:
Could not import extension autodocsumm (exception: No module named 'autodocsumm')
Makefile:70: recipe for target 'html' failed
make: *** [html] Error 2

[leandron]

cc @tqchen

[tqchen]

Thanks @leandron !