[Buildbot] #3474: Add Pull Request template on GitHub

[Buildbot trac]

#3474: Add Pull Request template on GitHub
----------------------+------------------------
Reporter:  rutsky     |       Owner:
    Type:  undecided  |      Status:  new
Priority:  major      |   Milestone:  undecided
 Version:  master     |  Resolution:
Keywords:             |
----------------------+------------------------

Comment (by rutsky):

 From discussion in IRC: we can add template with checkboxes of items from
 contribution guide (e.g.  http://trac.buildbot.net/wiki/PatchReview).

 Like:

 {{{
  [x] I ran validate.sh  / [x] I did a make hooks to install the git hooks
  [x] I added unit tests
  [x] I added relnote
 }}}

--
Ticket URL: <http://trac.buildbot.net/ticket/3474#comment:1>
Buildbot <http://buildbot.net/>
Buildbot: build/test automation