Added c++ reference.

Status Job ID Name Coverage
  Test
failed #306774548
allowed to fail
style-check

00:00:58

passed #306774547
test-suite

00:02:07

 
Name Stage Failure
failed
style-check Test
Collecting appdirs (from black)
Downloading https://files.pythonhosted.org/packages/56/eb/810e700ed1349edde4cbdc1b2a21e28cdf115f9faf263f6bbf8447c1abf3/appdirs-1.4.3-py2.py3-none-any.whl
Installing collected packages: click, attrs, toml, appdirs, black
Successfully installed appdirs-1.4.3 attrs-19.1.0 black-19.3b0 click-7.0 toml-0.10.0
$ black --check .
would reformat /builds/stephli/slithering-duck/bot/triggers/commands/cpp_ref.py
All done! 💥 💔 💥
1 file would be reformatted, 54 files would be left unchanged.
ERROR: Job failed: exit code 1