readme: document extensions

[glensc]

this project lacked description, what does the package contains.

tried to deduct the information from:

• https://raw.githubusercontent.com/jgm/commonmark.js/0.24.0/test/smart_punct.txt
• https://commonmark.thephpleague.com/customization/inline-parsing/#example-1---twitter-handles

they both seemed to be added in 0.1.0 with no docs:

• https://github.com/thephpleague/commonmark-extras/blob/9a3254bec56396e1f9c58911bdee2b38669b7a56/CHANGELOG.md#010---2016-01-13

[colinodell]

Thank you!

[glensc]

@colinodell uh, looks like there's duplicate extension

https://github.com/thephpleague/commonmark/tree/94305093262b9448b1aa8d40eb1178c2ad0508a5#league-extensions:

league/commonmark-ext-smartpunct ( 2 hours ago) - Intelligently converts ASCII quotes, dashes, and ellipses to their Unicode equivalents

[colinodell]

Yep I just created that today. My goal is to pull out the individual extensions and then deprecate this one.

[glensc]

what about strikethrough then? #18

I attempted to finish the previous PR.

[colinodell]

We can merge that once completed, pull that into a separate repo, and then decide what do with this one.

Right now I'm thinking we'll either deprecate / "abandon" this package or turn it into a meta-package which pulls in the things we've split off. I'm currently leaning towards that second option.