Add installation instruction on README

[kitchoi]

This PR updates the README for Pyface (also visible on PyPI page) to provide installation instructions for optional dependencies.

[rahulporuri]

LGTM

[corranwebster]

Probably should specify wxPython 4?

[codecov-io]

Codecov Report

Merging #797 (7e0774b) into master (4bccc3e) will increase coverage by 0.21%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #797      +/-   ##
==========================================
+ Coverage   40.67%   40.88%   +0.21%     
==========================================
  Files         508      508              
  Lines       27832    27832              
  Branches     4217     4217              
==========================================
+ Hits        11320    11379      +59     
+ Misses      16016    15979      -37     
+ Partials      496      474      -22     

Impacted Files	Coverage Δ
pyface/ui/qt4/tasks/split_editor_area_pane.py	65.41% <0.00%> (+0.59%)	⬆️
pyface/ui/qt4/console/console_widget.py	29.28% <0.00%> (+0.72%)	⬆️
pyface/ui/qt4/tasks/advanced_editor_area_pane.py	51.72% <0.00%> (+0.86%)	⬆️
pyface/wx/python_stc.py	10.36% <0.00%> (+1.21%)	⬆️
pyface/ui/qt4/tasks/task_window_backend.py	60.52% <0.00%> (+1.31%)	⬆️
pyface/ui/qt4/code_editor/code_widget.py	44.39% <0.00%> (+1.69%)	⬆️
pyface/ui/qt4/tasks/editor_area_pane.py	55.88% <0.00%> (+2.20%)	⬆️
pyface/ui/qt4/application_window.py	91.66% <0.00%> (+2.38%)	⬆️
pyface/ui/qt4/util/gui_test_assistant.py	80.53% <0.00%> (+2.65%)	⬆️
pyface/ui/qt4/window.py	73.87% <0.00%> (+2.70%)	⬆️
... and 8 more

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[kitchoi]

Ah... I tried to change 'wxPython' to 'wxPython4' via GitHub directly and GitHub did this change for me!

[kitchoi]

This adds the missing newline at the end of file, and I am going to merge away...