Update README.md

Default limit is 20.
https://github.com/mbleigh/acts-as-taggable-on/blob/1c257ad9f69e7c54fa6615160238ff929d23ba9f/lib/acts_as_taggable_on/tag.rb#L23-L24
Merge pull request #616 from harrykiselev/patch-1

Update README.md