Add coverage instructions to README

[k-collie]

See cython tracing and coverage

Need to add .coveragerc:

[run]
plugins = Cython.Coverage

steps are:

• add # cython: linetrace=True to top of cython .pyx files (note this also enables profiling, adding slight perf penalty)
• export CFLAGS="-DCYTHON_TRACE_NOGIL=1" before pip installing
• run coverage run -m unittest discover
• run cython --annotate-coverage coverage.xml idlbridge/_core.pyx or equivalent if this file moves (e.g. in 20)
• run coverage html
• browse coverage in htmlcov/index.html