Percent encoding for URI conforming to RFC 3986. Ref: http://tools.ietf.org/html/rfc3986#page-12

url_encode.rb

require 'liquid'
require 'uri'

# Percent encoding for URI conforming to RFC 3986.
# Ref: http://tools.ietf.org/html/rfc3986#page-12
module URLEncoding
  def url_encode(url)
    return URI.escape(url, Regexp.new("[^#{URI::PATTERN::UNRESERVED}]"))
  end
end

Liquid::Template.register_filter(URLEncoding)

How is this plugin supposed to be used while writing my template?

see https://github.com/mojombo/jekyll/wiki/Liquid-Extensions

so you would probably want sth like this:

{{ post.title | URLEncoding }}