docs: Update the markdown README file
docs: Fix the examples for ClutterText.set_font_name()

We shouldn't be using "pt": PangoFontDescription.from_string() assumes
points, and only accepts "px" for absolute font sizes.

https://bugzilla.gnome.org/show_bug.cgi?id=736826
Release Clutter 1.20.0
Post-release version bump to 1.20.1