Include "characterSpacing" style property at the docs

[andreybleme]

The style property "characterSpacing" can be used to add letter spacing to texts, as this issue demonstrates: bpampuch/pdfmake#869

I was not able to find it at the docs:
https://pdfmake.github.io/docs/0.1/document-definition-object/styling/#style-properties

I believe this should be added in this "Style properties" section.