Update README.md
Merge pull request #6 from datacentred/master

Mention custom domains in the README
Added support centre api
Add SupportCentreTopic class
Add SupportCentreArticle class
Those files should be ignored in a Gem by default (https://github.com/github/gitignore/blob/master/Ruby.gitignore)
Split find method in a find and a search method, to get more similarity with existing methods