Fix a typo in the Readme

It showed incorrect tag and the syntax was not in a code block
Import default markdown extensions from settings instead of readers

Closes getpelican/pelican-plugins#69

Due to change in pelican code, `import EXTENSIONS` was no longer
working.
Merge pull request #70 from talha131/fix-liquid-tag-issue-n-doc

Fix liquid tag that was broken due to change in pelican