Show logo in readme
This uses the github markdown feature that lets us use different logos in the dark and light theme, to avoid the text being invisible.
Seemingly this only works if the URLs point to github, and not external sites.
+![cocotb logo](/../../../../cocotb-web/blob/master/assets/img/cocotb-logo.svg#gh-light-mode-only)
+![cocotb logo](/../../../../cocotb-web/blob/master/assets/img/cocotb-logo-dark.svg#gh-dark-mode-only)
+
**cocotb** is a coroutine based cosimulation library for writing VHDL and Verilog testbenches in Python.
[![Documentation Status](https://readthedocs.org/projects/cocotb/badge/?version=latest)](https://docs.cocotb.org/en/latest/)
