Refactor Readme.md

- Fix indent in examples;
- Fix markdown lists;
- Add NLCST to related projects;
- Add signature of callback to API heading.
Add retext-emoticon to desired plugins in Readme.md
Add retext-frequent-words to desired plugins in Readme.md
Add retext-no-pants to desired plugins in Readme.md
Update changelog for 0.2.0
0.2.0