Adding issue and PR template to Theano repo (#4952)

CONTRIBUTING.md

+If you want to contribute to Theano, have a look at the instructions here:
+http://deeplearning.net/software/theano/dev_start_guide.html

ISSUE_TEMPLATE.md

+If you have any questions on Theano, please ask your questions in either [theano-user mailing list](https://groups.google.com/forum/#!forum/theano-users) or [stackoverflow](http://stackoverflow.com/) with the "theano" tag.
+
+Use issue only to report bugs or to ask for new features in Theano.

PULL_REQUEST_TEMPLATE.md

+You can make a pull request to Theano github repo in order to fix a bug or add new features.
+Once a pull request is submitted, [Travis](https://travis-ci.org/) is launched to verify the pull request
+given the current stack of unit-tests in Theano.