Note from [Apex Web Services and Callouts](https://developer.salesforce.com/page/Apex_Web_Services_and_Callouts):

> An Apex trigger can execute a callout when the callout is invoked within a method defined as asynchronous: that is, defined with the @future keyword. The @future annotation signifies that the Apex method executes asynchronously. For more information on the @future annotation, please read the documentation.

Also worth reading: https://salesforce.stackexchange.com/questions/5602/http-callout-from-triggers