MAINT: update README.rst.  Fix broken links and add "what is PyWavelets".
MAINT: update README.rst.  Fix broken links and add "what is PyWavelets".

Also reorganized everything a bit and remove a not very meaningful usage
example.

[ci skip]

ThomasA/pywt,aaren/pywt,grlee77/pywt,rgommers/pywt,ThomasA/pywt,aaren/pywt,eriol/pywt,kwohlfahrt/pywt,kwohlfahrt/pywt,ThomasA/pywt,eriol/pywt,grlee77/pywt,PyWavelets/pywt,rgommers/pywt,michelp/pywt,michelp/pywt,PyWavelets/pywt,eriol/pywt,aaren/pywt,rgommers/pywt,rgommers/pywt,michelp/pywt,kwohlfahrt/pywt