Added instructions to README on using an application from a different account

I added instructions on how to use an application from a different user account to get write privileges, so as to get around Twitter's restriction on having more than one account associated with a single mobile phone number. This allows you to use t to write to multiple accounts without having to have multiple phone numbers. Instructions are based on http://dghubble.com/blog/posts/twitter-app-write-access-and-bots/ .