README: Update, better usage, relative links

[chr5tphr]

In README.md:

• bring the Usage part up-to-date, with some better formatting
• add a small demonstration in Usage
• move Documentation to its own Section, so it can be found more easily
• remove statement about "interfaces can break anytime", the interface
  should be mostly stable now
• add another link to the documentation after Usage
• give more details in the introduction to the Example
• add a section Example Heatmaps, which shows example heatmaps for
  various supported attribution methods for VGG16 and ResNet50 produced
  by feed_forward.py
• add a sub-section about the documentation in Contributing with notes
  about building
• make absolute github links relative, such that they can point to their
  respective revision. They were previously absolute links for display
  on PyPI, which cannot correctly disply relative files.
• add badge for tests
• fix inconsistent capitalizations
• fix Pytest link pointing to Pylint

New images:

• add two images to show heatmaps for VGG16 and ResNet50

In setup.py

• define a function to fetch the long description for PyPI, which
  replaces relative links with absolute ones on github. This fixes the
  previous problem with relative links on PyPI, and will also display
  and reference files for the correct release, instead of always for
  master.
• due to python -m build by default copying all files for the sdist,
  and then building the wheel in the same folder in an isolated
  environment, the revison would not be correct within the wheel. The
  sdist however creates a PKG-INFO file, which will contain the
  long_description with the absolute links, which will be used if the
  git version cannot be used. The fallback is to use master, as done
  previously.