Add customize instruction section in README file

[ajhsu]

Since I started to figure how to customize my own filters, there have no explicit instructions described it in README file; Then I found that few issues (#83, #142) talking about the same question as well.

And I also found that @barryvdh did point that out within issue #83, said there are instructions written in the section of the config file, so I think it is worth to place a hint in README file; Which could save lots of time for other developers.

[coveralls]

Coverage remained the same at 21.388% when pulling 780459aec2fadb5d9bb588eaea65e3dbd63720ee on ajhsu:feature/add-customize-instruction-in-readme into 0a0784c6fb4d383bceebd7ee4f8a2dcac15df5cf on rcrowe:master.