Updated contributor how to guide

[Jake Farrell <jf...@apache.org>]

I've updated the contributor guide for recent changes we have been making
on the infra side which will now enable us to accept pull requests from
GitHub.

http://thrift.apache.org/docs/HowToContribute

Comitters, I have also added a new page detailing the necessary steps for
dealing with a GitHub pull request. Before accepting/merging the pull
request please ensure that the automated email from GitHub about the pull
request has reached the dev@ list.

http://thrift.apache.org/docs/committers/HowToCommit

If anyone has any questions please let me know
-Jake